I don't want to see more of Xehanort, but I think it'd be an interesting premise to follow in his footsteps. Let's say, Riku, Roxas, and Aqua go into a datascape version of Xehanort's journal for... some reason. They need to get information concerning where they think Kairi was sent but the journal has a cipher protecting it from being read visibly, I dunno, whatever. You'd certainly have some interesting personalities conflicting with each other and a variety of different play-styles to choose from (Riku perhaps a bit more conventional but with some dark powers to mix things up, Roxas with his unique dual wielding, and Aqua with her spell-slinger style), and considering all three of them were on the dark side at one point or another, there would possibly be some real stakes in terms of being tempted by the darkness that Xehanort has encountered in his travels.

Then in the real world that would mean you have Axel, Xion, Ventus, and Terra all under one roof. I can already see the drama now once the tension in the story heats up. Terra being overly protective of Ventus and Axel being overly protective of Xion could create a powder-keg moment which Xion might mistakenly set off not realizing that Ventus isn't Roxas or some such. It almost writes itself.