I'm actually beginning to think that it's Leo behind the door. From the way Veronica says, "I was hoping it would be you," and from her smile, it's almost the exact same way she smiled at Leo from the several of episodes he appeared in. Granted, she could be smiling like that for anybody but this is just the thought that had occurred to me after watching the sneak-preview during last nights repeat.
Now, don't get me wrong, I really want it to be Wallace behind that door, because he is Veronica's best friend. So I'm not ruling him out of the picture of speculation -- I'm just adding Leo as a possible candidate. The previews really are screwing with my head, but I have no time to complain about my speculations getting wired; the suspense is killing me for all that's about to occur in this new season!
Also, Charisma Carpenter was sizzling in that preview, might I just add that.