I think it's worth trying to give each race a consistent type of visual appeal aimed at a specific audience segment. For example, One could be cutesy, with fluffy (killer) bunnies and sheep, and maybe the buildings could involve cartoony flowers somehow. A different one could be female pin-up themed, with busty amazons or catgirls in bikinis. A third could have male pin-up equivalents of bishounen, probably accessorized with horses, roses, capes, and sparkles. A fourth could be creepy with skeletons, spiders, green poisonous gasses, and giant mosquitoes. And a fifth could be majestic - gryphons and dragons and warriors in elaborate armor.
This is an odd post to come across, but exactly the type of response I was hoping for. What if the graphics were different as well for the different sides you were on. For instance, if you play the fluffy bunny side, they think that the military side is a bunch of scary demons, they look like skeletons. But they also have "RAMbo" of the Sheep clan, and the Coo Coo Hatter of the Chicken clan banding to fight back, and claim areas. Then the human military doesn't see "bunnies" they see Rabid Rabbits of unusual size! (I don't think the exist, AAHHHH!)
I don't see my self as using this, but its an interesting idea. Imagine having two games, completely different server, and on one server, everyone thinks its starcraft, and on the other server, everyone thinks its bunnies fighting for carrots, a bunch of little kids playing, but behind the scenes, the games are the same, but the look and feel is completely different between the servers, with no clear connection.
I would also include that I like the idea from a different perspective. Like in the zerg, the HUD looks like flesh, and the Terran, it looks like metal/computer. I like this appeal from the stand point, that the HUD still operates the same in starcraft for each race. But what if a more biological based force had a more organic flow to the way your control them. for instance, with one race, you just point and click, giving commands, but perhaps with a league of robots, who are always centrally controlled, you can control every bit of there movement. down to taking over a machine and driving it with arrow keys, and mouse firing tactics.