Adding a character from FFXII seems ok.
Although it feels like adding a character from FFXIII would simply be pushing it, we know little about the game aside from a few of the main characters. I havn't even heard anything about the villian of the game, so if they do add something from FFXII well. I guess it'll spoil a few things.
Or they'll heavily delay the game, and this is one game that I can't wait to try out.
Why must JRPG female leads suck so bad?