Look, Maleficent said that she would help Sora only long enough to defeat the Organization and the Nobodies and that she would have her revenge. Seeing as how the Castle that Never Was is now hers (or at least, highly likely), I know of no reason why she should help and therefore cannot be a summon (summons are the GOOD GUYS!)
Second of all, Nomura's next project (after Final Fantasy Versus XIII) is NOT KH3!!! The next project will not have Sora as a major character (if at all) because - guess what? - Kingdom Hearts is Sora's story. This new project will be closely related to the KH universe, but it is NOT - I REAPEAT, NOT KH 3. As such, I don't think it will even the Kingdom Hearts name in the title, meaning there will probably be a new moniker.
That said, summons I'd like to see in the next KH game...
Cloud (he's just too f***in' awesome not to be! I mean, he was a summon in CoM, why not in KH3, in real-time 3-D with kick-ass special effects?)
Beast (I know he turned back into a human at the end of KHII, but c'mon, you think they would let such an awesome character go to waste?)
Kim Possible (I'm not sure if I really like the idea of a Kim Possible WORLD, but I want to see her in the next game in some way or another and this is the best one I can think of)
Roger Rabbit (again, not sure I buy into the whole Toontown world thing, but this would be awesome. He could drive around in Benny the Cab and smash into things and stuff)
Genie (probably the best summon in both games. His magical attacks just work)
Stitch (one of my favorite Disney characters, and his summon spell just sucked so much ass! They'd better do it right next time)
Adult Bambi (That would be cool, seeing him ram stuff, but I think he should also keep the health/MP drop from the first KH [yeah, I realize I don't have a whole lot of defense/restoration summons here])
Roxas (no originally a choice of mine, but it would be hella cool to fight alongside your dual-keyblade weilding nobody (in that cool-loking Organization gear of course). Would make for some pretty awesome battles I think)
King Mickey (Seriously, how cool would it be to fight alongside the great king HIMSELF for more than fve seconds.