You won't be able to play the FF characters in KH2.
I think you will only play as Sora and the BHK (Blonde-Haired Kid), who has very recently been named as "Rixas". You may be able to play as Riku as well.
And yes, it will be spectacular.

The trailers and info they have released at the moment could be a whole game's worth of stuff, but S-E have said in many interviews and reports that everything that has been said so far is only the tip of the iceberg, a little scracth on the surface of something far bigger and more spectacular than we will ever know.
I can't wait!
And you know what the best thing is? Not only will this game tie up any loose ends from KH1 and KH:CoM, but it will also give rise to new mysteries. Which means, dear people, KH3! Very possibly.