What if the Protoss like race were once peaceful pacifist beings with no plans for expansion, then they create a super ai that slowly starts taking over their minds and alters and upgrades their bodies and abilities.
Maybe this can be how they get their superhuman reflexes, cloaking and other things.
The ai's ultimate goal is to ensure the survival of the Machina race, as this was what it was created for.
Perhaps if a campaign happens, then there can be groups of their race that shun the ai and others that embrace it or think they can control it.
Possible faction Roster
Possible name (The Machina Miles - latin for machine soldiers)
Bonus: all units can repair themselves very slowly.
Cons: Most units are vulnerable to EMP damage.
Units and buildings:
Supercomputer: Produces tech workers and protectors.
Tech workers: fast moving worker this unit is vulnerable to raids but explodes upon death causing units around it to take damage.
Protectors: Cheap cannon fodder units, they are small robots with lasers.
Factory: Produces Cyborgs and Laser shooter.
Cyborgs: Augumented units, that can cloak and dodge damage occasionaly, only capable of melee attacks and periodic shock attacks.
Laser shooter: units fitted with light laser weapons, is very fast but fragile, good against air units and as a general ranged unit, pretty accurate due to its ai targeting system, can dodge damage.
Machine factory: produces airdroids and sensors.
Airdroids: Fast lightly armored flying Droids, especially good in large numbers, fragile but deal good damage.
Sensors: can take control of an enemy non organic unit, causing the enemy unit to self destruct and sense cloaked units in it's radius, rather expensive but powerful, can cloak for a short while.
Transport facility: trains heavy transports and ion ships.
Heavy transports: a slow heavily armored ship with limited attack capabilites, can transport units.
Ion ships: medium speed ship that uses a laser cannon attached beneath it, very effective against ground units but vulnerable to air units as it cannot attack air units.
Most of their units will be expensive compared to the two other races.