There is a very good idea!
So I help you to get ideas
The most obvious extreme and fun idea is the TYRANIDS!
This race will be very-very fragile,no any shooter units,just only meele,very-very fast units,many units. When you play tyranids you fell "Orks- meet Eldar"
or you play with the SC zergs.
More particular things:
-no generator resource,just only "blue" resource(sry I forget his name)
-creep like SC zergs(I don"t know maybe impossible to create this idea)
I think good if you use tyranid mode models and taskba,just need new icons new voices for unit voices AND weapons voices and a little texture fix for units.