O_O'' Don't hurt me!
OK, I have a few suggestions from the top of my head. I know they won't be used but I just want to share my ideas.
Anyway, there should be a unit in Terran, it's just like the Dropship in the Alpha/Beta stages of SC, it bombs stuff, but the drawback is, it has a weak anti air weapon and the bombs are fairly expensive, but it's still fast anyway. A Terran Falcon could be the name...
For Protoss, there could be a fast vechile. It rams into infantry with a psionic thing on front to kill them take off half of their HP (eg Marine has 28/40, but the HP would then be 8/40). It has two drawbacks. It has veeeery low HP and once it runs into a non-biological unit it explodes, but it does completely destroy the vechile that's rammed into. It wouldn't do much to structures, so it's more like a defense weapon. Well, I'll call it the Protoss Basilisk, I guess.
The Zerg's new unit could be the Zerg Montalisk. If the names at the start of the 'lisk' Zergs have a meaning then it should be something different. It would be a long range shooter. But, it would shoot something like the Reaver. But, it can go over cliffs and water. The bad things about it would be that the unit things would be expensive and fairly slow.
Plus, an upgrade could be Robotic Implants for the Dragoon that makes them faster.