To answer the question rules-as-written:
Wherever the DM thinks appropriate. There appears to be no answer beyond that (especially nothing as specific as a table of default destinations) using official 3.5e materials - 3PP d20 material or earlier editions may be another matter.
For a more useful answer, we need to go beyond rules-as-written (which after all is the whole point of having a DM in the first place.) The best answer therefore depends on the style of game being played, of which there's infinite variation but here's a few ideas:
The narrative answer:
Wherever adds a good story twist, which probably means a populated area with NPCs to interact with. If it's the PCs first visit to that plane, a large city would probably be a good way to introduce them to the plane, as well as providing a useful home base. On the other hand, a small settlement would provide for more intimate interaction with local NPCs and lead to easier story hooks.
The conjectural cosmological physics answer:
The 3.5e Planar Handbook (p151) mentions planar breaches between planes, where the dividing line between two planes has worn thin. It follows that this would be a likely entry point to another plane, as the barrier between the two would provide the least resistance at these points. Planar material could be flowing in or out of this breach depending on the relative density of the planes concerned.
The Hollywood movie answer:
As mentioned in the opening question, there may well be a specific location on the destination plane that is related somehow to the point of origin of the travelers. Travelers from the plane of water may arrive in the middle of a lake or ocean, for example, while those coming from the plane of fire may arrive in a volcanic area.
To the best of my knowledge, there's no definitive list of these, and in many cases it's necessary to tweak the idea to something that would be appropriate on the destination plane - arrival from Mechanus may seem difficult to do if the destination doesn't have clockwork, but a ring of standing stones shaped like cogs feels like a good fit.
The random chance answer:
The DM grabs whatever maps or charts they have of the destination plane (sketching something out if necessary), makes up a grid reference scheme if needed, and rolls dice. Having a few easily adaptable generic maps at hand (with some locations that can easily be tailored to any plane) can be useful here. Tables would work equally well if preferred.
The murderhobo answer:
The PCs arrive at the entrance to the lair of planar monsters that need to be killed, because... they're not currently dead enough, and they have valuable stuff to take.
The "Teach them a lesson"/"My players are masochists" answer:
(Thanks to Zachiel for - unintentionally - reminding me to include this)
They arrive in the worst possible location. Covered in flames on the Plane of Fire, drowning on the Plane of Water, inside solid rock on the Plane of Earth, between two rotating cogs on Mechanus, five thousand feet above one of Arcadia's layers, etc. You'll probably never run into the problem of unspecified destinations ever again...
As you have said you are willing to consider Spelljammer rules, I can pull a few quotes in for you that might help clear this up.
From Spelljammer Core Rules: Book 2, Concordance of Arcane Space, p. 17
Clerics in Space: A cleric may not regain spells above 2nd level while in the phlogiston. This is because he remains out of direct
contact with his deity.
A cleric who enters a foreign crystal shell is similarly restricted.
The only exceptions to this occur when a cleric's deity (or a similar
power) is recognized in the foreign shell or the cleric casts a
successful gate spell, affording his deity access to the foreign
shell.
Any god or power is considered "recognized" within a sphere if he has
worshipers and/or an organized church anywhere within that system.
This does not mean that a PC cleric can move into a shell and
automatically establish a base of worship for his god. For a deity to
be a recognized power in any shell, he has to have an established
base of worshipers somewhere in the system.
Additionally, the same rulebook adds the following spells on page 22
Contact Home Power (2nd level Conjuration)
The user of this spell establishes a tenuous link through the Astral
Plane between his present location and that of the power he venerates.
This link permits the priest to regain his spells as if the god is
recognized and venerated within the sphere.
The duration of the spell is one week, as long as the cleric remains
in the same crystal sphere in which he cast the spell. Passage into
another plane (even the ethereal) will break the spell.
and
Detect Powers (2nd level Divination)
[This spell] allows a cleric to determine if there are friendly gods
and/or other powers within a crystal sphere so that he may recharge
his higher-level spells within its borders. It also reveals if the
god(s) revered by the cleric are worshiped within that sphere.
In this case 'friendly' is defined as 'same basic attributes and portfolio' or possibly 'same god by a different name.' As two examples...
-Paladine and Bahamut are the same god, but known by different names and roles in two different spheres.
-Reorx and Moradin are two distinct gods (from Krynn and FR respectively), but they are sufficiently similar that a cleric of one could receive spells from the other.
So, if you take these pieces of information into account, it gives you the following answer:
When you are on a different world within the Prime Material Plane, you may only regain spells up to 2nd Level unless
1: Your god has an established base of worshipers on that world, or on another world within the same Sphere. This implies that the other gods in the system don't actually get a direct say in the matter...though they could certainly stamp out 'heretical religions' if necessary.
2: You use Detect Powers to locate a friendly deity (or your own) and refresh your spells via them.
3: You use Contact Home Power on a weekly basis to 'phone home' and keep that connection open so you may reload your spells.
4: You use Gate to connect directly to your deity's domain and refresh your spells from them directly.
Now, I do not have access to the Planescape books, so I am not 100% certain that this isn't contradicted somewhere in those books...but if you are willing to abide by Spelljammer rules, that's how it works.
Best Answer
There have been various suggestions for mixing dragons and creating hybrids, from calculating average values for the hybrid (adding up the values of the parents and dividing them by two etc) through applying a colour theory of sorts (a blue and a yellow parent would produce a green offspring) and other solutions to applying unofficial templates (such as I linked here).
I'd suggest checking the links for a more thorough discussion. :)