With the artwork for EMorvana up we can be reasonably sure we'll be getting an epic to go along with the 3pic in Gargantuan (and a few more shinies besides according to Matt) so since no one else has started a speculative thread of speculation I figured whyy not jump on in.
Out of our five warlocks who haven't gone epic yet I think we can rule out Naaresh as being too new, and probably Rasheth for similar reasons (plus not seeming like a "go-epic" kind of guy), that just leaves Zall, Xerxis, and Mordikaar.
Morvana suffered a significant plot setback in Domination, but I'm not sure how much that helps us for prognostication; Xerxis suffered serious injury, Mordikaar got shuffled off to the deep desert somewhere, and Zaal (along with Hexi) knows we have serious bad juju headed our way. My money is on Zaal as being in the best position to figure heavily in the Gargantuan fluff.
As for the 3pic, I'm betting on 3Morghoul. I think the safe money is probably on 3Makeda, but there's a decent chance of a new character beast for the 3pic and EMorghoul needs the boost more if it isn't a Companion.