They should implement a double-bladed light saber! This would give reason to implement a dismemberment physic for every part of the body. J/K :D
In all seriousness, I'd like to see more effective, high-level weapon types that aren't swords. I don't know what Blizzard's obsession with swords is - maybe it has to do with some deep seeded connection they have with Highlander and their quest for dominance - but aside from Starcraft, they tend to favor swords in all of their games. . . Frostmourne attests to this.
scythes that can disembowel opponents
pick-axes that can gut enemies
flails that swing around wildly and bash monsters' skulls in
double-bladed axes (speaks for itself)
hammers that have a chance to rip and cause bleeding effect with the sharp end
glaives and other pole-arms that have a chance to penetrate through to multiple monsters nearby (much like the Amazon's passive ability from D2)
relevant throwing weapons that aren't just a gimmick early on, but are effective throughout the game
torches that burn enemies and provide better lighting
crossbows that penetrate at close range, while bows are a faster attack speed - maybe even add different reagents to customize the damage type, such as fire and oil
swords would obviously be easier to thrust, thus quicker
I know the new mechanics relating to weapon type are a stretch (not even sure how well some of it would display in an isometric game), but it'd add a whole new perspective on choice of weapon and a change of game play depending on that choice.