you could go to town on this really different ships with smaller hulls and less weapons
challenge modes to stop incoming missiles by shooting them down, single player style
a bonus 'saucer' flying through the middle to up your score both get to shoot at it a la space invaders bonus saucer type thing
separate health on turrets might be interesting
easy addon to shop might be a hull repair for some serious hours of play against your chums :)
depends how much you want to expand on the original which is pretty damn good and easy to play the lack of single player does make it a few games before you put it down