It should come out soon cross..

FenixMarco- Did you borrow a theory from cross? Nah, just kidding.. its a decent theory. :-p

Hades probably should be the one to unseal Zeref, seeing as thats his goal.. I wonder if he will end up teaming up with ivan(laxus father.. makarovs son) to achieve it.. I think itd be awesome, anyway.. Gray would have to fight ultear, i think.. The fact that shes urs daughter is too great of a connection for them to fight someone else.. Also, then Laxas could get a rematch against erza.. or maybe fight natsu again.. Dunno what gajeel would do.. he could be on either side, kinda like laxas, though.. Gajeel is supposed to be a spy, right?
Although, it seems more likely that mashima will split both of those arcs up.. I just think itd be awesome to connect raven tail and grimoire heart.. so much potential together, i think. O.o