well i read something a while ago that said FF XII wasn't out when Dissidia was first concieved/production started, so they had to leave it out...i still believe, like you, that there will be no unlockable characters (besides the 20 main ones), but i really wish there were some

however, i disagree with the characters from XI and XII or even XIII because they have no nostalgic value, as you mentioned, and i just think they would be dead weights in the game...on the other hand they could get you really psyched for the release of XIII, but, as mentioned, i doubt there are unlockables at all, let alone ones from games that haven't even been released (i refuse to acknowledge 'Luso' in Tactics to be an example of where square has already done this )