I doubt Archons would be there, and MC won't be there, unless it is like the MC in WoW, temporary and it prevents you from acting. Dark Templar would be awesome because they'd be permanent stealthed Rogue units with shields.
Hydralisks would rock, and would probably be the single most played Zerg unit.
Lurkers are my personal favorite Zerg unit, and they would be really interesting in an MMORPG, due to their unique attack. They would probably be dropped or made enemy only. I wonder how burrowing would work...
Rogue Units:
Ghost: Stealthy attacker, but focuses on ranged rather than melee combat.
Dark Templar: Melee unit with shields and permanent stealth.
Zergling: Fast and agile, and can burrow to hide.
Warrior Units:
Firebat
Zealot (or Archon if they have them)
Ultralisk: Probably the strongest armored units in the game, with the attack to match, definitely the Zerg tank.
Mage Units:
The closest the Terrans had was the Science vessel, and I don't think that'd work.
High Templar
Defiler/Queen
Priest Units:
Medic: Only real healer in the game, so we'd have to add one for the other classes.
Hunter Units:
Marine
Don't think we had a ranged Protoss ground unit, unless the Archon is allowed.
Hydralisk