I'm working mostly from the straight Kingdom Hearts side of things. Spelljammer stuff may show up moar later. I really do want MindFlayers to be inexplicably a part of this setting.
- What is the deal with my cleric's religion?
There are many, MANY gods throughout the cosmos, each situated in an individual Crystal Sphere. Olympus, for instance, has the Greek gods, as one might expect; while Port Royale (or whatever I end up calling the Pirate And Mermaid Themed Land) has a variety of pagan sea gods and spirits (Davey Jones, Calypso, etc). Some places have nature spirits. Honestly, the possibilities are as endless as the worlds themselves. Hell, your cleric might even be an atheist.
The one religion which tends to show up on multiple worlds is that of the Light / Kingdom Hearts, a remnant from when the worlds were all one world. On Radiant Garden / Hollow Bastion, this was essentially the state religion, though religion there worked more like it does in His Dark Materials, ie, it was married quite closely to the sciences. I'll have a whole post on this alone because RG/HB deserves its own worldbuild considering its importance to the setting as a whole.
- Where can we go to buy standard equipment?
This depends largely on the world; most worlds do have various shopkeeps who sell standard gear, but sometimes that gear varies. A world like the Deep Jungle, for instance, has very little magic, so most equipment will be mundane. Agrabah, on the other hand, has a lot of unusual magic as while magic isn't strictly commonplace, it's still easier to find than on other worlds. Before the Fall, Radiant Garden was a land where magic and technology were seamlessly integrated, but, well, the Fall.
That being said, Moogles have traveled to and pioneered most worlds, and even sell Moogle Gems, crystals that let you access their shops from any location.
- Where can we go to get platemail custom fitted for this monster I just befriended?
Moogles! In addition to being multi-world first class businesscreatures, Moogles are also talented armorsmiths and craftsmen.
- Who is the mightiest wizard in the land?
By and far that's Maleficent, Queen of the Unseelie Sidhe. Do not fuck with her.
On the side of the good guys, probably Master YenSid.
- Who is the greatest warrior in the land?
Hercules, winner of the last ten years of tournaments at Olympus' famed Coliseum.
- Who is the richest person in the land?
Scrooge McDuck, thanks to his inexplicable cross-world ice cream empire. He also manages Moogle Enterprises.
He really does have a giant bin full of money. Attempt to break in at your own peril.
David Xanatos is another power player, but his world is currently cut off from other worlds, and as such he doesn't have the same multi-world spanning empire that Scrooge does.
- Where can we go to get some magical healing?
That really does depend strongly on the world you're in. Potions are actually common on almost EVERY world, because they can be made from common ingredients, even from various goos that Heartless drop when they die. For more powerful stuff, though... Olympus has some good clerics, and Traverse Town has a number of refugees from distant lands.
- Where can we go to get cures for the following conditions: poison, disease, curse, level drain, lycanthropy, polymorph, alignment change, death, undeath?
See again: Traverse Town is probably your best bet, being a ramshackle refugee town for people from many distant worlds.
Being cured from being a Heartless or a [REDACTED] is much, much harder, and can't be done by any known magic.
- Is there a magic guild my MU belongs to or that I can join in order to get more spells?
DEPENDS ON THE WORLD THEY'RE FROM. Radiant Garden used to have a very powerful University, but it was blown up pretty damn good. Merlin, YenSid, and the Fairy Godmother all know each other in passing and are happy to teach travelers clearly on the side of Good their spells, but they aren't part of any formal organization, also YenSid is a taskmaster and kind of an asshole who will make you carry ten billion buckets before he teaches you one level one spell.
- Where can I find an alchemist, sage or other expert NPC?
Traverse Town is your best bet.
- Where can I hire mercenaries?
Again, Traverse Town. The Coliseum is also a good place for wandering dudes who like to punch things.
- Is there any place on the map where swords are illegal, magic is outlawed or any other notable hassles from Johnny Law?
I'll get back to you on that, but mainly: you don't want to be going and waving your stuff around in Port Royale.
More importantly, there are a large number of worlds who know NOTHING of outsiders, even if they DO know about the Heartless threat. According to the people of That Town Which Totally Isn't Disney Town I'll File The Serial Numbers Off Later you're not supposed to meddle in the affairs of other worlds beyond protecting them from unnatural threats; this is also the primary creed of the ancient Keyblade Order... but the Order's been dead for thousands of and seriously who the hell is going to enforce this stuff?
- Which way to the nearest tavern?
Traverse Town has some great ones; so does Port Royale.
- What monsters are terrorizing the countryside sufficiently that if I kill them I will become famous?
HEARTLESS FUCKING EVERYWHERE
THEY COME IN DIFFERENT FLAVRZ EVEN
- Are there any wars brewing I could go fight?
- How about gladiatorial arenas complete with hard-won glory and fabulous cash prizes?
- Are there any secret societies with sinister agendas I could join and/or fight?
[REDACTED]. Joining might not be something you actually want to do, of course. But, well, if your character dies horribly, there might be an opening...
In addition, Maleficent and her League of Villains are always looking for fresh blood to assist them in their ultimate quest.
- What is there to eat around here?
DAMN BRO depends on the world. Olympus and Agrabah though are especially known for their fantastic cuisine.
- Any legendary lost treasures I could be looking for?
Well there's these rumors about this thingy called a Keyblade.... though apparently only certain Chosen Ones can wield them.
There's also the legendary Magic Lamp of Agrabah, which contains an all powerful Robin Williams, I mean genie.
- Where is the nearest dragon or other monster with Type H treasure?
The nearest dragon is Maleficent.
What did I say about Maleficent up under question 4.
Other monster types likely to have serious phat lewt: the Greater Heartless tend to have lots of random junk floating around their bodies, even if that's only solidified concepts that can be used in synthesis. Big Bads of each world also tend to have lewt, though they are not always monsters (Ursula the Sea Witch, Jafar, etc tend to have excellent lewt)