I still place my bets on a polearm or staff, maybe switches into a bow.
Blizzard stated, as others have said, several things on the next legendary.
1)Its not for tanks.
2)No melee daggers.
3)It will have a much more narrow use than the caster staff of 4.2
Dragonwrath is usable by 5 classes. Based off of the 'much more narrow' statement, I'd take a guess at 2 or 3 max classes using it. 4 is not much more narrow. Since casters just got Dragonwrath and str melee just got Shadowmourne, this leave rogues, ferals, hunters, and shamans. Hunters and ferals could share a polearm/staff(could possible switch to a bow on right click), and rogues/shamans could share mace/axes. Yes, hunters could share as well, but due to them not having any 1h enchants this expansion, its not likely, unless they add one.
Lastly, feral has never had a legendary, rogues had glaives for quite a while(and 2 at that!), shamans had 1 way back in vanilla, and hunters had the VERY short lived end-of-bc bow. Maybe I'm biased, but...I bet it will be hunter/feral.