I like this idea. Thought about something similar. No new unit is needed, though. We already have them in the game:
Transports. They just need to be allocated some attack power...
As it is now, Transports are basically useless (besides for ferrying troops). You could just delete them from the game and nobody would miss them actually.
But if transports had some firepower properly allocated to them, transports could actually become something quintessential to the game concept.
They should be allocated minimal offensive attack power during fig phase, more attack power during ground assault phase, and a lot of defensive attack power when stationed on a planet during all three phases (bomber phase, fig phase, and ground phase).
If this happened, Transports (or maybe more aptly named 'trankies' ) would be left stationed on newly captured planets with the other ground and figs. This would help players get lasers / portals built on planets in enemy-controlled systems.
They'd also be useful for players in defending a portal from getting busted by a larger fam's main attacker. They'd have to be stationed on a planet, though, to provide the defensive attack power bonus. Otherwise they could be used to defend all portals (too big of a defensive advantage).
If ever capital ships were introduced (as I have already shown would be as easy as Pie) 
, 'Trankies' would be instrumental during assaults to capture enemy capital ships.
Initially, players could think of them as transports with gun turrets during fig phase, so that on offence they can shoot-down a defending fig or two if lucky. When they hit the ground and unload their troops, however, they 'dig in', transform into tanks / artillery / anti aircraft units that can hit ground units, figs, and even bombers when lucky. They'd also have to become at least somewhat targetable by enemy bombers to be fair and maintain game balance.
