We got Auron once so I guess the chance is there.

Lightning as supporting guest character!

I'd also like to see all important characters and Final Fantasy characters be a part of one great final battle - like the one in Radiant Garden but actually towards the end and on a bigger scale. I would not mind seeing Sephiroth, Kefka, Kuja or others showing Xehanort he is not the only villain who actually is a problem and at least fighting him for a moment or be immune to him or whatever.