I feel like a single game exploring all those storylines will take way too long.
1. You've got Sora, Yozora and running around in Shibuya, which in itself will take a long time - There's so much to explore! We might dive a little into Luxord's deal, Magia and Aegis, the Nameless Star...
2. You've got Riku, whose dreams we're exploring. Maybe we've got FF characters coming along for the ride...? This alone will take up at least half the game.
3. You've got Kairi, who had been asleep getting her heart searched and may or may not be awake in this game. Best case scenario is she comes along with Riku to find Sora. Worst case scenario is she stays asleep with AtW and his apprentices staring over her. And maybe Naminé.
4. You've got Terra, Aqua and Ventus (and most likely Chirithy, too) in the Realm of Darkness. Anyway, if we're exploring this storyline, Ventus will undoubtedly (hopefully) find out AT LEAST A LITTLE BIT ABOUT HIS PAST. Maybe we might somehow see Vanitas again? Maybe Darkness will come into play? Who knows?
5. You've got Roxas, Xion, the Twilight Town gang and possibly Lea, Isa and Naminé in the memory department.
6. You've got Goofy, Donald and Mickey exploring worlds that Sora has been to - And every world is HUGE. Are they really going to search every nook and cranny for Sora? You'd need a game exploring just this storyline alone.
7. You've got Ephemer, Skuld and maybe others like Brain... Maybe somehow Strelitzia...? And their doing... whatever it is. Might perhaps delve a little into Subject X.
8. You've got the Foretellers, Luxu, MoM, AND don't forget Maleficent and Pete... with the ominous black box.
9. You've got Marluxia and Larxene who've now turned back into their Somebodies, Lauriam and Elrena. Seeing as they're villains, we most likely won't get to play as them, but we might get a cutscene or two.
10. You've got Xehanort, Eraqus, all the Dark Road characters whose names I've forgotten... I mean, we most likely won't see them again but like, there's probably important characters and lore and stuff in Dark Road that we'll find in another game... You never know with Kingdom Hearts
11. You've got these... other characters. Demyx and his "keyblade legacy." We... might or might not see Riku Replica again? We might even see our Player character. Somehow. They might just go with the default male Unicornis player.