Given that Naruto and Dragon Ball both got secondary stages, I think it's reasonable to speculate that Kingdom Hearts will get one too. Unfortunately, I don't think that we'll be seeing any Disney-based stages in the game, as it would be too tough to narrow down a single world, and it might also give Disney cause to rain hellfire upon our heads. Yes, that means you too, Disney Castle. So, anyway, here's every original location I could think of from Kingdom Hearts, arranged in order of how likely I think they are to get the spot next to Twilight Town.

The World that Never Was
The most likely candidate in my opinion, given the general bias towards KH2 representation and the fact that it's been widely speculated for years, plus the fact that it looks pretty cool. Would most likely be based on the castle as it appears in KH2, perhaps with some elements drawn from KH3D - though I wouldn't be upset if a stage was based on the Dark City instead.

Castle Oblivion
A fan-favorite, often speculated alongside The World that Never Was, and shares a good deal of aesthetic with that area. Would also be a handy way to give some representation to the handheld side of KH, even if that is CoM. Ugh.

Keyblade Graveyard
A very important place in the lore of KH. It's plenty different from Twilight Town yet fits right in with many of the other stages in the game already. Would likely have a simple layout and kickass tunes.

Hollow Bastion
Another favorite from the series. I'd expect it to be based mostly on KH1, but all versions of the world that have appeared throughout the series may provide some interesting stage elements. Could also be a really big stage.

Destiny Islands
More peaceful than most KH worlds, but definitely a major location, and a good contender for a beach-themed stage. Might change from calm to stormy through the battle, which would be a really nice visual effect and possible stage hazard.

World of Chaos
Now this could be interesting. Who doesn't love a good brawl on a giant battleship made of darkness with a big shirtless dude watching and possibly interfering? And the music, as always, would be sweet.

Realm of Darkness
World of Chaos not dark enough for you? Let's just go straight to KH Hell instead! It's definitely got the "distinct from Twilight Town" part down. Might even find Aqua here.

End of the World
I realized while writing this up that I don't remember anything about the way End of the World looks, other than the bit that's pulled straight from Destiny Islands, and the World of Chaos, which I gave its own spot.

Traverse Town
Overlaps a bit with Twilight Town - I think the devs would probably look for some more diversity in picking stages from the same universe.

Radiant Garden
It's a nice enough place, but Hollow Bastion would almost certainly take precedence.

Daybreak Town
Quite a shame, really. Daybreak Town would look glorious sprited in SSF2 style. Unfortunately, the KH mobile game isn't likely to make much of an impact on the series' representation in SSF2, and we already have two stages based on towns set in front of a giant clock tower, one of which is Twilight Town.

Station of Calling
This was actually planned to be in the game really early on, appearing in one of the ancient trailers from the very beginning of the project. I don't think the layout lends itself well to a good SSF2 stage unless a good deal is added on. Would look nice, though.

Land of Departure
Doesn't have a whole lot to distinguish it as a stage, and it's not exactly the first location that comes to mind when you think of KH. Gets points for potentially having Dark Impetus as a track.

Datascape
Gets the bottom spot on this list for being a) from what's considered the most forgettable game in the series, and b) kinda lame compared to the other entries. Sure, the Bug Blox could make for an entertaining stage hazard, but realistically, this one just isn't happening.

I Want To Believe that Mute City (Melee) or Big Blue could return as past stages, but within the limitations of Flash that seems unlikely. Mute City (SSB4) is more likely, and if F-Zero does get a second stage, I'm guessing now that this will be it.

Game and Watch probably won't get a second stage unless it's a previous Flat Zone.

With three characters or more and just two stages, Kirby is ripe for a few new ones. Fountain of Dreams is of course a popular pick. Beyond that, I think we could see a new Kirby stage based on one of the more recent games - the official Smash series hasn't had any from a Kirby game newer than Super Star.

Not familiar enough with Star Fox to pick out good stage ideas, and I'm not positive the series will get any new ones either, especially since past stages like Lylat Cruise and Venom seem difficult to replicate in Flash.

Metroid is pretty well represented by the Crateria and Phase 8 stages, but it would be neat if Brinstar or Brinstar Depths made a comeback, or if a stage from the Prime games showed up.

Don't know anything about Chibi-Robo, but the Desk stage seems like it represents the series well enough.

Already talked at length about Kingdom Hearts. Nobody cared.

PAC-MAN probably won't get a second stage unless PAC-LAND returns, but I don't expect it to.

Tales is another series I don't know much about. It would be nice to see a stage that represents a game other than Symphonia, though.

Once again, know nothing about Bomberman. There's like a bajillion games to pull stuff from, right?

Guess how much I know about Bleach. Go on.

One Piece has boats in it or something? We have Gangplank Galleon already, but surely there's something else...

Palutena's Temple is almost certainly off the table. Skyworld could appear, as could Reset Bomb Forest. Based on the other stage choices in the game, I think the devs might choose something...underworldy?

If Venus Lighthouse ever actually gets added, it's unlikely that another lighthouse will appear as a Golden Sun stage. There's a few other locations that could work out - the Elemental Sanctuary, Apollo Ascent/Apollo Sanctum, and Crossbone Isle come to mind. Of course, Golden Sun might stick to one stage instead, which is fine by me.