I would bring back Arthas. But the reborn Arthas i imagine Blizzard bringing back is none of the previous Arthas we see in WoW and WC3. I see Bolvar bringing back his soul (which was trapped inside Frostmourne) but since Arthas body no longer as a earth (the LK removed it) it seems the only way to bring back him is as a undead. Anyway, this new Artha i see Blizzard could bring back, would behave pretty much like a cripple, both in the physical aspect and in the psychological aspect, but he would be in fact Arthas the Paladin, or at best a combinationation of Arthas the Paladin and Arthas the DK (but with a soul) and not the soulness DK we see in WC# the Frozen Throne and in WoW.