Personally I think the Horde needs its own magic group, and even after the war winds down it wouldn't make sense for the Horde to be eager pile back in there, or for the rest of the Kirin Tor to be eager to have the guys who backstabbed them twice in town again.
Now, while Northrend-Dalaran is frozen in time to the Wotlk expansion, in content after MoP I think it'd be cool to have separate mage groups, not just the Sunreavers/Blood Elves to counter the multi-racial Kirin Tor, but toss in some Forsaken and maybe some orc and troll mages too.
What do you think?