It looks like only Sora, Riku, and Mickey have major things going on right now. Mickey really is the true tritagonist of the series after Sora and Riku. Like everybody else has said, the other groups have things to potentially keep them around and busy.
Wayfinder group- Ven's Darkness, Union X stuff. Terra and Aqua can fit in here as Ven's surrogate parents. Lauriam will probably tie in with this story as well. Kairi might be in here as well bringing the Wayfinders up to 4 characters.
Twilight Town gang- Subject X. Lea and Isa are mainly tied to it, Roxas and Xion will probably tag along for Axel at least. I just want to see how they differentiate Isa from Saix. I don't expect HPO to do much of anything besides being cheerleaders or maybe doing at home research.
Then there's Namine who will likely do what she's been doing since COM ended. Show up, magically fix a problem, say some exposition stuff, then disappear for the remainder of whatever story is taking place. They could get creative and have her scan Ventus, so long as it doesn't take a year to do it.
But yeah, this is looking MCU like with such a huge cast. That's not even taking into account Ansem's group and the Restoration Team at Radiant Garden. I don't think they all need individual games, but there is enough chores to go around it seems. But it's different when asking if they should still be prominent though.