If you want aliens in Battlefield, there's always the
Galactic Conquest mod. Pretty fun if you get the hang of it, though the blatant lack of parachutes is friggin' annoying.
I think its developer, Prolific, vanished off the face of the omg interwebs a looong while ago, by the way.
I was never that fond of "Vanilla" BF and definitely can't
stand the newer Battlefield's.