I think that its Lenalee
I could imagine her being the heart because of her innocence protecting her and that the innocence "Apocryphos" Said to Allen, that especially the heart was worried for him (and at the time Lenalee was completely broken.)
Or both Allen and Lenalee are the heart.
Both Allen and Lenalee are exorcist, but Allen has a dark side (The 14th)
There are to sides to the war and what if the collide? I mean Allen and Lenalee could be the red threat.
I could imagine that Lenalee has to fight Allen, but that she can't kill him and he can't kill her, because of their feelings towards toward each other. And together they find a solution to end the war.
That could be one of the reasons Lenalee and Allen share the same dream.