I have had problems trying to make a total conversion for starcraft that replaces the Terran and Protoss units and buildings with new zerg units and buildings. In order for the player to make all the units in staredit just have them start with two of each races drone (Normal Drone=Zerg Drone, Flying Drone=SCV, Psi Drone=Probe)

New Unit Ideas

Flying Drone=SCV
Air Unit, Worker
Weapon: Stinger
Damage: Low (Air,Ground)
Makes Zerg Buildings

Rageling=Marine
Ground Unit
Weapon: Needle Spray
Damage: Low (Air,Ground)
Abilities: Rage=Stim Pack

Infernolisk=Firebat
Ground Unit
Weapon: Flame Breath
Damage: High (Unarmored) Low (Armored)
Abilities: Rage=Stim Pack

Swarm Keeper=Medics

Shadeling=Ghost
Ground Unit
Weapon: Mantis Claws
Damage: High
Abilities: Camo=Cloak

These are just a few ideas if anyone wants to help create the mod I would be happy to help with any storylines that may come up.

A topic for posting unit ideas already exists:

http://www.staredit.net/topic/6956/

And there are more simple ways to have a player use multiple races (e.g. firegraft).