Background
As I mentioned in this question, I have recently started watching the Avatar: The Last Airbender animated series and am inspired to improve the otherwise underwhelming Way of the Four Elements monk archetype. To this end, in addition to the suggestions proposed in the other question, I have been expanding the list of Elemental Disciplines to include other spells that I think suits the themes of airbending, earthbending, etc. (especially the elemental spells included in Xanathar's Guide to Everything, originally from the Elemental Evil module).
For spells like erupting earth, flaming sphere, tidal wave, wind wall, etc – in other words, spells that are within the range of 1st-5th level spells – I can simply copy what has already been done for the existing official Elemental Disciplines with regards to discerning how much ki it should cost to cast these spells.
Excluding those that have their own rules rather than allowing the casting of spells (e.g. Fangs of the Fire Snake), the disciplines that cast spells all cast spells that are in the range of 1st-5th level spells, and all follow the formula of "ki points required = spell level + 1" (except for Rush of the Gale Spirits, which only costs 2 ki but lets you cast gust of wind, a 2nd level spell, but it's a weak 2nd level spell, so that's probably why it's slightly "cheaper" than the other disciplines for casting 2nd level spells).
Proposal
I think high level earthbenders, firebenders, etc, should be able to do truly Avatar-level powerful bending once they reach tier 4. More specifically, I want to come up with some disciplines that add spells of 6th level+ that a 17th level Way of the Four Elements monk can take, but without this being broken. Spells I'm considering include:
- bones of the earth (6th level spell)
- earthquake (8th level spell)
- fire storm (7th level spell)
- investiture of X (6th level spells)
- move earth (6th level spell)
- tsunami (8th level spell)
- wall of ice (6th level spell)
- whirlwind (7th level spell)
Again, to reiterate, all of these disciplines would be available only to tier 4 monks, meaning they'd all have the (17th level required) prerequisite. Also, at time of writing, I am not currently considering including any 9th level spells such as meteor swarm, so if excluding 9th level spells helps in any way, that works for me.
Question
If I were to include disciplines that allowed the casting of 6th level+ spells, following the formula I derived (so 6th level spells would cost 7 ki, 7th level spells would cost 8 ki, and 8th level spells would cost 9 ki), would this still be balanced? Would the ki cost need to be increased because of the fact that these are "higher level spells"?
Given that ki can be replenished on a short rest, would I also need to add additional restraints on these "higher level spells" such as only being able to cast them once per long rest (like how certain warlock's Eldritch Invocations have that restriction, such as Sculptor of Flesh, even though it still uses a warlock spell slot), or would the ki cost be enough on its own?
By "higher level spells", I'm referring to the fact that 6th-9th spell slots are fewer in number, as pointed out in Mindwin's interesting (although off-topic) question. Given that apparently the designers though that higher level spells should be cast only sparingly compared to 1st-5th level spells, this is my reason for being wary of allowing my monks to cast such high level spells potentially multiple times a day due to ki replenishing on a short rest.
Best Answer
The short answer is “No.” The long answer is as follows:
What you’re proposing is basically making the archetype become a higher powered pseudo-caster. So let’s take a look at what the end-game modifications would create.
The monk is essentially a martial class, which means they will do fine in close quarters and, at a base level, can do fine with just Attacking. While not as hardy as say a Fighter, it certainly isn’t as spell-reliant nor squishy as say a Wizard or a Warlock. As is, the Way of the Four Elements Monk could be considered a half-caster, much like a Ranger or Paladin which can eventually gain 5th level spells. However, the upside to the Way of the Four Elements is that they can regain the resources used to cast their spell during a Short Rest and their resource is more flexible.
Your proposal turns the Monk into almost a full caster which will regain their resources on a Short Rest. So let’s make some comparisons:
Compare the Monk to a Ranger and you’ll find that with your changes, the Monk will do about as well as a Ranger when it comes to just martial fighting, but their spellcasting suddenly becomes much more potent with 6th-8th level spells meaning now they’re wildly outclassing the Ranger.
Comparing it to a Warlock, unless that Warlock has specifically built for it, the Monk should be better martially. Not only that, but the main stats utilized for the Monk’s martial and spellcasting abilities will give the added benefit of additional AC which the Warlock has no equivalent of. Finally, the Warlock is the closest in spellcasting resources to the Monk in that they regain their spell slots during a Short Rest, however this is not true of the Mystic Arcanum (6th and higher level spells) they unlock at higher levels which they can only cast once per Long Rest. So overall, I’d say that your proposal make the Four Element monk almost equivalent to a Warlock on a spellcasting basis, though i’m hesitant to really say that considering no class has the ability to cast Firestorm or Earthquake twice per short rest.
Finally compare it to the Wizard. The wizard can also regain some resources on a Short Rest, but they’re limited to spell slots of 5th or lower level. So basically we run back into the problem that the Monk is regaining their uses of these much more powerful spells than a full-on caster would.
While the Way of the Four Elements Monk is indeed written underwhelming, the way I solved this was by actually giving them passive features in addition to what they already have. While this doesn’t emulate an avatar-like character, it does at least make it more fun and viable.
If you want something a little closer to an avatar-like archetype, I supposed you could add a few 6-8th level spells with the stipulation that their cost to cast is (the level of the spell + 2) Ki and that once cast, they can’t be cast again until the monk finishes a Long Rest.
On the other hand, you could also just make a character avatar-like and role play them to fit the role. You could play something like a multiclass Monk/Druid or Monk/other spellcaster. Heck, you could even just play a straight up Druid, Sorcerer, Cleric, or Wizard and role play them as though they were a Monk of sorts.
Additional Information: A few of the passive abilities I add onto the Way of the Four Elements monk include things such as learning Cantrips from the Druid spell list at 3rd level, being able to make an Unarmed Strike as a Bonus Action whenever casting a Cantrip as an action at 6th level, or being able to make an Unarmed Strike as a Bonus Action whenever casting a spell or using an Elemental Discipline at 17th level. Basically just little things that kind of put them up alongside the Eldritch Knight archetype.
A couple others that may be iffy depending on how high-powered of a game someone wants to run includes abilities to recover Ki, such as recovering 1 Ki whenever reducing a creature to 0 hp. This is just to facilitate the use of Elemental Discipline more often, especially when throwing weaker enemies at the party.