I registered specifically for a few unit ideas I had, which I felt necessary to post. :P I'm an RTS player and I even used to play C&C back when Westwood still made them. I've played a few of petroglyph's games, too. So anyways, here are my ideas for units. Some are very C&C or StarCraft inspired.
Cobra - Lightly armored, quick hovercraft or amphibious wheeled vehicle that is armed with a missile rack. It is cloaked when idle or moving and can cross water areas.
Lifter Truck - Giant truck that has an aircraft bay on its back. The VTOL craft on its back can take off and attack a unit that is far away. It must, however, return to the truck and land to regenerate its ammunition and heal. Should the VTOL get destroyed, it can be rebuilt.
Chameleon - A small mobile vehicle that can deploy to either project a cloaking field or a shield.
Colossus - 2-legged mech armed with a cannon (Think of the "Titan" mech from Command & Conquer: Tiberian Sun). It can walk over other vehicles and ignore them completely, unlike tanks, which have to go around enemy vehicles. It has a much wider range and can "entrench" or "armor up" (It curls itself up) to protect itself from enemy fire (perhaps become highly resistant to damage, or deflect a certain percentage of damage back towards the enemy), but becomes immobile while doing so, and cannot attack any enemy units in this mode either.
Striker - Beefed up infantry completely encased inside a powered armor (Think of the Terran Marine from StarCraft), armed with a powerful gun (Gauss Rifle? Railgun?)
Prometheus - Lighter, quicker variant of the Archimedes. It is a hover tank, and can cross water areas as a result.
Mobile Defense Platform - A threaded utility vehicle that can deploy into a point defense structure, which can arm itself with anti-armor, anti-infantry or anti-aircraft turrets. (I know, ripped off FoC)
Assault Platform -A large combat vehicle whose weapons can be customized. It is essentially a more mobile MDP, with more health and armor, and requires more points to deploy. (Think of it as a sort of super-unit) It has 2 hardpoints, each being customizable with a certain weapon.
Some kind of amphibious jeep/scout vehicle would also be interesting, I think. Could be for either factions.