Why make a separate branch?
The Navy gets the Star Destroyers.
The Air Force gets the TIE Fighters.
The Army gets the Stormtroopers.
And the Marines will somehow get all the credit.
LOL! Seriously, it could be a multi service force under a seperate name. We have seen the development of SpecOps forces that draw from two or more services. The Space Defence Forces could be such a critter (one organisation encomapssing personnel from the different services).
LOL!