For GDI I'd like to see the return of the "jumpjet trooper" from tiberian sun, you could use the jump and crouch keys to rise and lower, the mouse could be used to look around and change direction. The weapon could be a chain gun.
The wolverine from TS would be a cool sight on the battle field as well, with its twin chain guns blaring away.

For both teams I have this idea:
Add two becons to the purchase menu, one costing around 700 credits and one costing 900.
When you place these, a plane or chopper goes over an inplacement is parachuted down its type depending on the ammount you paid.
The 700 credit one is a cannon inplacment (the ones in single player) and the 900 credit one is the missile inplacment.
If you read the info in single player about the inplacments it says they are parachuted down.