Traveller would do nicely for a Renegade Legion RPG. There was a Renegade Legion RPG (Legionnaire), but I didn't find it that great a rule system. (Hmmm, I may even still have my copy of that somewhere - one to add to the eBay pile I think).
ACTA would make for a great fleet action version of Leviathan (the cap ships game), or for that matter High Guard can handle the right sort of feel (huge ships with massed weapon batteries and spinal mount weapons of terrifying destructive power).
Battlefield Evolution system could be tweaked to provide a tactical ground game - or Mongoose could licence and reprint the original Centurion game, which was the best of the RL boxed games.