Those are actually some really interesting theories~!
I'd hack up a lung laughing at the reactions to everyone in the series if it was Cross, that'd be amazing ♥.
The Lavi & Komui theories are really quite awesome, I'd actually never considered them to be the heart before....Hmm (mind goes into overload at the possibilities.)

I'd imagined the heart would be Allen or Lenalee but it kinda seems a bit too obvious :|