My favorite AR, and probably the rifle I currently shoot the most is an 18" .223 Wylde SPR style rig. With a decent trigger, mil/mil scope and a few good loads it's just a lot of fun to shoot out at extended ranges. If you don't have an AR to fill that slot and you have the range to stretch out, something like that might be worth a look.
I'm currently building a 16" 6.5 Grendel, as a compact, lightweight ~250 yd medium game hunting rifle. I've used a 6.8 SPC for the same role before, and it worked well, I'm expecting very similar performance.
Another option that might be interesting is a 12" 6.5 G pistol or SBR. Reports on the short Grendels online seem to be pretty positive overall and it certainly would make a very compact package, especially if you run suppressors.