The key here is to note the renewed interest in Ultima Online once it introduced the 3D client and made an expansion or two.
If they ported D2 over to the Warcraft 3 engine, then spiced some existing content up, added a new character class and maybe some cool side quests or a side act, many people would be foaming at the mouth to get their hands on it.
Chew on this:
Male and female counterparts for all classes
New dazzling 3D spell effects and better model representations for equiptment
A new class
Revamped quests to make unused portions of the game desirable. (ref: stony tomb, the cave, the hole, the later sewers, etc.)
A brand new act filled with side acts. Maybe another lesser evil like Andariel.
A few new skills for each class to replace unused ones.
Suggestions: a better high end Druid elemental spell; another damage vine with more power or the ability to summon more than one poison vine; a Nova like AoE melee attack for paladins or barbs; remove the lightning from death sentry; make a high end assasin trap that shoots her stars (poison??); a high end bone shield for necros that works like the assasins blade shield; a lightning shooting hydra for sorceresses; rework necro skeletons to be like druid wolves; give amazons a better high end poison javelin attack; make guided arrow and magic shot or maybe most of the bow tree work with javelins; an assasin Nova spell with shuriken or perhaps like frozen orb with throwing shuriken (no cold effect); a druid elemental spell with thorns sprouting from the ground like the monster in the first part of act 5
Where do your thought lie?