Ok, so it has been quite some time since someone looked at this thread, so I reckon no one will even read this. Never the less, I've given it a bit of thought and have come up with my best idea for a forth race.
So this race is really two races, half and half. But you can basically play a whole game with just the main race, and not even use the other. The main race is huge robots. Long ago they were made by the Xelnaga, but forgotten. They recently wake back up. Of course the Xelnaga are now dead, so they plot revenge against the three races that they think might have killed them. They style themselves as the new Xelnaga, and take on parenting child races. One race in particular have come to think of the robots as gods, and fallow them everywhere and fighting with along side them. I'm thinking a cross in between Fremen and Kroot. So their collective goal is to destroy the races they think killed the Xelnaga, and to take control of the universe to rule over all the lesser races. Starcraft takes place in only one sector, so I can only assume, scattered all over the place, are lesser non-warp capable, stuck on their home world.
Robots: clunky, bulky, but still powerful units. Don't want to rip off any hive mind, so each bot has its own personality. They are ruled by the larger bots, who maybe used to be in other bodies. I think design wise something like the fallout robots. The theme of the whole army is expansion. Building and units can be made quickly, numbers of built units can be stored and then made in an instant. They fight from mobile bases, because units can make other units. Only a few building are made by the worker unit, the rest are made by the building themselves. So they can build something to block and expansion, but they can't proxy a rax very easily. about half of their units are "mother" units, in that they can make most of the other units. So lets say there last unit, in the same tier as the mother ship, can make just about every unit, is a death ball in and of itself. I talked about that replicator supply building, here is a better idea: for a price, it spawns 8 Gremlins. They can attack, but they can also repair other units and building. Whey they repair, they east up some of there own health. Gremlins can't repair other Gremlins. I have a lot of other unit idea's but that's all for them for now.
Natives: more of a contently evolving and collective race. They have only one structure, the village, that itself is the CC and rax and everything else in one. This structure can also move, it takes a bit to pack up, then turns into a hoard of the natives, that can either move to a new location or attack as a huge hoard. the only other building is a supply building, a farm of course. The natives have their own worker unit, its a group of kids. They do all the harvesting. Then you can select them to train. They go back to the village and train into a warrior unit. then this unit can go back and train into a stronger unit. so basically they have have one unit, that keeps growing over the game, but of course you can have as many of them as you want. Also most upgrades effect every single unit. Lets say, double attack, or invisibility, the list goes on.
Over all I think this is so different then the other races it will work. Of course balance work would need to be done. I have a lot of idea for different units. In terms of ranking against the other races, the robots would have the health of protoss, while the gremlins and natives would be weak like zerg. of course lots of strats could be used, but i think the main one would be, using the robots as your main army, and having the natives sneak around to all the expo's and suck up all the resources.
Ok that's my idea. Sorry if my writing ain't no good haven't written in awhile. I think this is something this site should come together and do. Once we've figured out what units we want, it would be super easy to make.