I really rather the lot of them be retired & never appear again in the series. Not shitting on them here, I just think KH4 needs to be more of a fresh start & dump a lot of the baggage.
Unfortunately that's already not gonna happen.
I like most of the people in these trio but I would be happy if they dont play any big role anymore (or at least not all of them). But it seems that at least Axel and Isa will play a role because of the unknown person. But imo that might not mean that the other two will follow them. Maybe Roxas and Xion might just want to live a normal life for once, but at the same time would Roxas truly sit down with Sora being gone? Will he fully trust Riku with bringing him back?
Ventus will probably be important to the next story because of him coming from the age of fairytales. Terra and Aqua not so much but would they sit back and let Ventus on his travel alone?
So theoretically they have reasons to be part of the next big KH game but it would just bloat the list of characters again..and we dont even know how many more new characters we get.
Maybe this could be at least partially solved if they get a KH game before KH4 that shows them and let them be playable as a sent of?