Aqua, Ventus, and Terra are three of KH’s most important characters, taking part in all of the background events that influenced the entire series. Their fates have yet to be resolved, though.
Despite a dream version of Ventus making itself known here and there, the real Ventus remains asleep in Castle Oblivion. Terra is still trapped inside Xehanort, and Aqua remains lost in the Dark Meridian.
Terra’s body is rather significant for plot purposes, so it’s likely something will happen to him, but whether that’s a good something — like getting his life back — or a bad something isn’t certain.
The same goes for Ventus, but with a twist. If his heart is with Sora, how can Ventus be revived? And if Vanitas has something to do with it, will Ven be mostly darkness?
Square Enix already said Aqua isn’t playable in KH3 and isn’t a major character; instead, the developer teased that she’s fallen under Master Xehanort’s influence. With the yellow eyes and all, it’s not a surprising turn of events.
Aqua remained trapped in the Dark Meridian, while others like Mickey, Sora, and Riku could travel away from it, likely because they had conquered the darkness in their hearts.
Having failed to save her friends can only have strengthened the darkness in Aqua, making her both an easy and a pitiable victim for Xehanort’s machinations. Whether she’s one of the 7 Guardians of Light, after all, remains to be seen.