You do get to use both Oblivion and Oathkeeper, which both float away from your hands, as if they have a mind of their own. And then it maxes you out on everything, meaning you're, I would assume, 100 on all stats.
That's actually pretty crazy, but awesome none the less. I must argue that I don't believe it's BHK, if he is Sora's shell, then wouldn't a fusion mean that this would remain the true form? It doesn't make sense.
I, personally, believe it's just Donald and Goofy still. Only more powerful, optional doesn't mean other characters, it's just your same team with better options and greater abilities. But that's just me.