Or they could do a FFXIII style party system, where you control the person that is the most relevant to the plot at that point in the game, until later where you can freely switch around team members, including who leads and you play as.
If they use an actual party system I hope it starts of with Sora/Riku , with Kairi/Lea being a seperate story arc untill they join up later (in FFXIII terms, think Lightning/Hope and Sazh/Vanille), and then, in order, Aqua, Ven, Roxas, (perhaps Namine), Xion and then, post-storyline, since he's a darkness, Terra.
I left out Donald and Goofy on purpose since I don't see em being very helpful anymore in the next games.
wow I wish I though f that. The final fantasy 13 format would work well. However sora of course wuold have the longest periods of gameplay times. I doubt aqua and ven would join our parties. There too powerful, and I don't want a nerfed memeber tagging along, like we had riku in at the end of kh2. Depenind on what point we bring them back mabey we can have short solo adventures with them before everone reunites.
Roxas... doesn't seem sad or regretful. He was content. Can they bring him back if he doesn't want to come back?