How about an elemental race like:
air - entire race is based on flying units and levitating buildings. Extracting sunlight from sources and gathering lumber by lifting entire trees in mid-air.
fire - race based on destructive fire elements, spreads lava (like blight) and has many burning and immolating features, obviously...
earth - a race made from the earth itself, rocky buildings, vines, has trees for allies, uses rains, mists, earthquakes etc.
water - a race build over shallow/deep water that has units dependant on liquid, probably units regenerate faster on water, has tsunamies and other watery specials.
Remines me of what I was forking for for contest 5, although it was never finished.
Except I made the roles being:
Unalinged-generic builder that makes all the different buildings
Air-fast moving scout units, stealth and general harrasment units (and harrasment hero.)
Fire-ranged units and siege. massive damage but easy to kill. (orbital bombardment style hero)
earth-melee tanky guys, serve as "meatshields" of the army. (super tank hero)
Water-support group of the army, spellcasters that buff and debuff. (with support hero)
Then upgrades allowed you to splice elements into units, making a unit multi-elemental to give it new abilities (like splice fire into a golem-like creature to make infernal-like creature, or splice air to the fire dragon siege unit to allow it to take off and fly at high speed but he needs to land to attack again, stuff like that.) it was pretty cool and now I talk about it I think I'll get back on it, just for fun.
EDIT: NOOOO! its GONE!
I only got the half-baken forgotten race from contest 6 (that I abandoned because it didn't inspire me enough...only the "stealth" hero of that race (Fabricator) was actually fun.)
Not sure I got the energy to do it all again
Anyway, whatever the theme is, as long you allow a few imports I am ready to begin even today. and as now I am no longer in the army, unlike during contest 5, I can actually spend more then three hours a week on it, so I WILL get things done. (because I cant work with only blizz models. it limits my mind too much. I need at least a FEW stuff)
ALSO-I want to add that I am AGAINST paired contests. it really blocks you unless you can find a good partner that you have good communication with. some of us prefer to work alone.