I am partial to support units. They can change the game in interesting ways. Consider the following:
1. Mobile shield (think Gungans) - Small damage mitigation aura/field. % miss chance as opposed to just more damage soak.
2. Buffer (or debuffer)- Unit that could give a slight overall increase to units in it's aura (attack bonus, defense bonus, range bonus, etc.) or give a negative effect to enemy units (slow down movement speed or attack speed, or any number of possibilities)
3. Poisoner - Unit which can apply a damage over time effect to enemies.
4. Mobile Radar. - Huge attack radar sight (but no change to the actual player view)
5. Hider - Unit that generates stealth. Can be a unit that can hide 1 other single unit or can be a field. I am doubtful on this one, though, because it seems like a deliberate, determined decision was made by the developers to NOT include the stealth mechanic in this game.
6. Painter - Unit which can, at very long range, mark a specific target for an airstrike or some other high damage attack.
7. Controller - Unit that can make an enemy unit switch sides for a period of time.
These would all probably be T2 level units. Their weakness is that none of them can directly attack.
Just some ideas.