I have a match grade AR already. I've also thought about getting an M4, but I've always wanted a pistol caliber carbine. I could get one of the AK's out there, but they are generlly too small for me to shoot comfortably.
The receiver idea is a good one. I built my first AR, it'd be fun to do again. I'd still like a 9mm carbine for plinking though.
Maybe I'll make a list of guns I'm missing tonight and prioritize it.