This is undefined under the rules
Unfortunately, the rules don’t explicitly say.
Some argue that you simply cannot choose that class again, and therefore lose that benefit unless you have some other class to choose
Some argue that you should look for epic progression for class you’ve finished, and barring that should make one
Some think you just get as much as you can (improved Caster Level), and ignore the rest (additional/better spells per day/spells known)
Looking at the strict RAW, the first point seems to me to be mostly wishful thinking; the rules don’t ever actually say that you cannot choose a class that’s “finished,” and for that matter they don’t exactly even make it clear that you cannot take more levels than are listed in the table (look up some of the arguments about combining Hellfire Warlock and Legacy Champion, where this is hashed out in great detail).
Thus, the latter two seem more likely. I don’t like the epic rules and don’t think it’s a good idea to give out those effects early (or at all <.<
), so I favor the last point. Furthermore, the special requirements of epic prestige classes lead me to believe you cannot get them before 21st level, no matter what you do. And unless the Epic Level Handbook is explicitly in play, those rules may not exist at all, so there may not be such a thing as an epic anything.
Note that, even if you do get the epic progression, it very likely won’t actually give you anything aside from improved caster level, since most epic progressions don’t and instead use bonus feats to grant you improved spellcasting. Even epic bonus feats require you to meet requirements, and that includes being an Epic character which you are not.
Anyway, ask your DM. Since your DM has already allowed Wizard 5/Ur-priest 2/Mystic Theurge 8, even though you don’t actually legally qualify, he’s clearly a very different DM than I, or any I have ever played under, so maybe he’ll let you get 10th level spells before you could ordinarily get 9ths. Why he’d allow that, I have no idea; I certainly wouldn’t. But based on what he’s already allowed, he just might.
No, or at least pretty much everyone agrees you can’t
As Thomas Jacobs says, there is no hard-and-fast definition given by the books for “spellcasting class.” Various non-book sources (FAQ, Customer Service, and so on) have repeatedly denied the ability to advance mystic theurge with another prestige class that advances a “spellcasting class,”1 but such sources are not officially a part of the rules (per errata documents).2
That said, a community consensus does exist,3 and it is just about universal:
A class that advances spellcasting is not, itself, a spellcasting class; a spellcasting class is one that has its own spells.
So, with that in mind:
assassin, bard, blackguard, cleric, druid, paladin, ranger, sorcerer, and wizard are the core spellcasting classes, even though some of them are prestige classes.
arcane archer, arcane trickster, archmage, dragon disciple, eldritch knight, hierophant, lore master, mystic theurge, and thaumaturgist are not, even though they require or advance spellcasting.
The classes in the first list could all be advanced by mystic theurge, and the arcane ones (assassin, bard, sorcerer, wizard) could be advanced by anima mage, but the classes in the second list cannot be advanced by anima mage, even the ones that advance arcane magic.
There are two prestige classes, legacy champion in Weapons of Legacy and uncanny trickster in Complete Scoundrel, who can advance any other class. These prestige classes could advance mystic theurge and therefore advance two other spellcasting classes, one arcane and the other divine.
I cannot find the precise location of these mentions, but I do find reference to it (and to the fact that this stuff keeps getting deleted).
As StevenO mentioned, we hashed this argument out ages and ages ago (circa 2004, I think - as soon as the Mystic Theurge was previewed pre-3.5, in fact, as it was the first dual-caster out there). However, most of the threads discussing this were destroyed in assorted board purges, and I don't see the discussion in the official FAQ either. We did get a Sage ruling on this and, unusually, a unanimous response from customer service (believe me, that is bizarre!), along with a solid consensus from die-hard rules lawyers who were more adept with mathematical logic than I (see, for instance, our threads discussing the world damage record, most of LordofProcrastination's dirty tricks, or any serious thread discussing either Pun-Pun or his challengers, which frequently delve into cardinality, infinity, and philosophy of time as well).
You can find all errata for 3.5 on Wizards’ official page for 3.5 updates; each errata file opens with a section explaining how errata works and defining primary and secondary sources. At best, the FAQ and CustServ are secondary to literally everything else, but most don’t grant them even that title. See here for more on what is wrong with the 3.5 FAQ.
Quick examples of community consensus:
• Here’s one thread, with one voice of disagreement
• A second thread with no disagreement, but an unanswered call for evidence
• Another thread with no disagreement (and an alternate interpretation of why this doesn’t work)
• Plus I managed to find one from Wizards.com, rather difficult these days (scroll down, it’s the last four posts of the thread).
• The Mystic Theurge Optimization Handbook makes no mention of the possibility, but lists “No Special Abilities for advancing levels” as a disadvantage; this would not be an issue if you could just progress mystic theurge with some other prestige class. It also does not use such an option in any of its example builds, which it doubtlessly would if it were considered valid.
Note in all cases that this is taken as obvious; the consensus is really strong and most don’t realize there is any ambiguity about it.
Best Answer
"You told me to go back to the beginning. So I have."
For PCs, I suggest following the Epic Level Handbook (EL) (July 2002) on Monsters as Epic Characters that, in part, says
If you change the heading to Monsters as Epic Player-characters and Cohorts, these rules are fine and playable. Here's how they shake out for the question's characters:
The natural werebrown bear human paladin 7. The creature possesses 7 class levels and gained 7 HD from those class levels. The creature has 6 animal HD as a werebrown bear. The creature has LA +3 as a natural lycanthrope. The creature's ECL 16.
The creature advances normally for 4 more class levels until it's ECL 20 with 11 class levels. When it advances another class level after that, the creature becomes epic, its 12 class levels making it ECL 21.
The locathah cleric 3/wizard 3/mystic theurge 7. The creature possesses 13 class levels and gained HD 13 from those class levels. The creature has 2 humanoid HD from being a locathah. The creature is unfairly burdened by a wholly unreasonable LA +1 as a freakin' locathah. The creature's ECL 16.
The creature advances normally for 4 more class levels until it's ECL 20 with 17 class levels. When it advances another class level after that, the creature becomes epic, its 18 class levels making it ECL 21.
All creatures accrue full benefits from class levels until they possess 20 class levels. Starting with their 21st class level, creatures receive the reduced benefits from class levels as per the EL's Table 1–1. HD that are from sources other than class levels always yield their normal benefits.1
If you, dear reader, don't care how I reached this conclusion, and you don't care about NPCs with LA (and their wealth!), then you really don't need to read any more of this answer. If you're the DM, though, remember this answer when you're stuck for how much gear that magmin Rog19 should have—that's addressed below.
As always, LA's the problem (especially with NPCs—but it doesn't have to be)
LA impacts only a handful of game elements. It affects how much XP a creature needs to advance a level. It affects when the creature becomes epic (therefore when it can take epic feats). And it affects the creature's gear. DMs usually don't worry about the XP for NPCs, but DMs tend to worry about NPCs' epic feats and gear. However, while the game predicates several rules on ECL, the game doesn't provide LA for all its creatures, and many unavailable-as-PCs creatures should have significant LA by the game's own evaluative methods (as Savage Species (Feb. 2003) describes) if the DM is expected to check an NPC's ECL. It's as if the game forgets that not every creature has LA. For instance, Monsters as Epic Characters, in part, says
Further, Monsters and Class Levels, in part, says
To make matters worse, the Dragon #310 (Aug. 2003) Wizards Workshop column "Sage Advice: The Sage Goes 3.5: Official Answers" includes this exchange:
Finally, worst of all, as part of an exchange that seems exclusive to the Main FAQ, there's this:
The DM must determine what to do about this big, angry pile of rules with regard to NPCs. Compare the creatures below.5
Obviously, there're substantial differences here. For consistency and verisimilitude, the DM must make a decision. I can imagine three options; readers may have more.
The DM rules that every creature has LA and assigns LA — creatures LA +0 or higher. The DM can use the methodology from Savage Species, harken back to the never-updated-for-the-3.5-revision Dragon #293 article "Monsters with Class" (52–5), or just make up a number, but every creature that's LA — is at least LA +0 instead. PCs and NPCs now have parity, but the DM must rewrite a significant number of monsters because they're woefully underequipped.
The DM rules that some NPC creatures have LA and some NPC creatures do not. The DM rules that an NPC that has LA — is effectively LA +0 and also rules that an NPC with at least LA +1 factor its LA into its ECL. PCs and NPCs that are playable as PCs (i.e. creatures that have LA +0 or more) now have parity, but the DM must still gear up a few underequipped monsters.7
The DM rules that only an NPC's class levels plus the NPCs HD from other sources count for toward its ECL, ignoring LA for NPCs. The big picture is that PCs that possess LA, uniquely for their kind, get access to epic feats earlier than comparable NPCs.8 There's no parity between PCs and NPCs, but the DM only has to rewrite a few monsters.
I've experimented with all of these, and I always end up at #3. LA is primarily for players and their cohorts, so the DM gets to ignore it, and I'm done.9 Using #3 means the aboleth mage, hound archon hero, and celestial charger aren't eligible for epic feats, but the frost giant jarl is. That sounds good to me.
Now let's talk about gp. Because #3 ignores LA, the value of NPCs' gear—for the most part—drops precipitously, but ECL remains a factor for NPC wealth according to the Monster Manual. The NPC aboleth mage's defeat still yields 130,000 gp in gear, but the NPC hound archon hero's down to 100,000 gp in gear, the NPC frost giant jarl's down to 265,000 gp, and the NPC celestial charger's down to 59,000 gp. But while this is less gear, it's still too far much. For instance, an encounter level 17 encounter like with a lone CR 17 NPC aboleth mage or NPC frost giant jarl should only yield an average of 36,000 gp (DMG 51).
So I suggest just ignoring the above from DMG, the MM, and the FAQ with regard to NPC gear and using instead Monsters with Classes that, in part, says
And I suggest using NPCs with Treasure that, in part, says
Using these rules in combination means that the NPC aboleth mage may have double standard treasure for its CR 17 plus its 16,000 gp in gear for its 10 class levels, the NPC hound archon hero may have standard treasure for its CR 16 plus its 21,000 gp in gear for its 11 class levels, the NPC frost giant jarl may have standard treasure for its CR 17 plus its 9,400 gp in gear for its 8 class levels, and the NPC celestial charger has no treasure for its CR 13 plus its 7,200 gp for its 7 class levels. This is more treasure than the creature's typical treasure—obviously, and especially if you're hunting unicorns—, so the DM still must be careful, but it's not the incredible lootslosion that defeating NPCs yields with the alternatives.
1 Epic creatures with class levels that also possess HD from other sources may have base attack bonuses and saving throw bonuses that are impossible for epic creatures that possess only class levels to achieve. First, that's exactly what the MM actually does, and, second, PCs that have HD from sources other than their class so rarely benefit from them that I'm okay with that.
2 Although the MM implies otherwise, it's only a choice to equip with gear a monster that has class levels insofar as obeying any rule is a choice. That is, monsters that have class levels always have NPC gear.
3 The Sage here is Skip Williams, the primary author of the Monster Manual for Third Edition. About a year later, with Dragon #223, 3.5 revision architect Andy Collins starts providing Sage Advice.
4 Reading that parenthetical cost you 2d6 SAN. Go ahead. Read it again. That parenthetical means rewriting every HD 21+ monster. Seriously, no printed creature follows that.
5 There're more creatures to dissect. If you're soldiering on alone, don't rend your garments: The premium Monster Manual (2012) changes the sample elite vampire's LA +8 to LA +5.
6 "What could a unicorn possibly do with all that gp?" you ask. The celestial charger has Int 13 and Wis 27, so I think the answer is, "Spend it well and wisely."
7 This idea has merit, but it makes some NPCs eligible for epic before other comparable NPCs solely because the former's LA +1 or more and the latter's LA —. For example, a CR 15 troll Rgr10 (monstrous humanoid HD 6, class level 10, and LA +5) can have an epic feat, but a CR 20 wraith Wiz15 (undead HD 5, class level 15, and LA —) can't. The DM should make this clear before the campaign begins as it directly impacts how players judge NPCs.
8 Not incidentally, PCs will also have significantly more gp than comparable NPCs. They'd have more anyway because they're PCs, but still.
9 A creature with LA +1 or higher that becomes a PC's cohort may find itself eligible for epic feats. It also may lose that eligibility if its association with the PC ends. I suggest the DM doesn't have a cohort take an epic feat until the cohort would be eligible to take an epic feat as a normal NPC (rather than as a cohort), but I suspect the entire issue is unlikely to arise.