Nothing wrong with folks wanting an Inifinity Wars-styled story that's brimming with cameos, but it just isn't my cup of tea. I'd personally want something more focused and character-driven.
It saddens me that this is the new benchmark for ensemble epics as it's such bad form, when there are nonetheless so many prominent and excellent examples of how to make the most of a large cast in literature, stage, and even anime and manga. You really don't have to look far for rewarding, densely characterized fiction going back centuries.
Personally, I'm not interested in seeing characters act or interact just for the sake of it-- what's important is that every storytelling decision, whether it surrounds a character's action or inaction, should be internally justified in following a thematic throughline. If you know you only have so many opportunities to give life to these characters, you just have to make every minute meaningful: Nomura's had more than enough time to plot out these arcs so that they resolve organically, so now it comes down to the choices he makes as a director.
For instance, one of the 4chan "leaks" discussed a potential confrontation between Aqua and Larxene and Vanitas. I think that has some really thrilling potential that functions on an emotional level anyone can follow: Larxene knows her way around Castle Oblivion, Vanitas needs to find Ven so that he can force a reunion with him, and Aqua would naturally be there to fend both of them off. The blending of personalities is excellent, as Larxene and Vanitas both exhibit a similarly savage, selfish and unpredictable pattern in their characters and Aqua is firmly set against them as another's protectorate. At the same time, it adds some questions to the equation which actually have answers we want to know: we think Aqua can defeat Vanitas in a one-on-one, if only just; would she able to hold her own against two similarly vicious aggressors; consequently, has the journey through the Realm of Darkness made her weaker, or stronger? Similarly, can Larxene and Vanitas, two people with potentially diverging agendas and explicitly hostile tendencies, be expected to work in real partnership to achieve the same goals? There's an argument to be made for some of the clashing symbolism, too-- Aqua as a representation light and water, serene and quietly forceful, facing off against darkness and thunder, crushing in their paired volatility. This is how you bring characters from disparate storylines together in a moment of conflict, but of course you can extend the logic to any of the various kinds of drama a long-running serial inherits from its antecedents.
It'll be unfortunate if a story like KH, where the characters all have generally compelling origin stories and journeys to draw on, can't manage to make the most of that foundation.
The posts about how 7 Disney worlds are enough and the game is massive, didn't age well.
Speaking of savage.