OP, I think you'll like that 6.8 when you finished, especially if you are a hunter. I used an 18" 6.8 (in MOE garb no less) to take two of my deer last year, and it worked great.
In case you load, I was using AA2200 (a great powder for the 6.8 ) with CCI 450 primers to push 110 gr Accubonds to 2660 fps with no problems.
As for my next AR build, well, I just ordered an M&P 10 with Magpul furniture and Troy sights, so my "build" will probably consist of me slapping a scope on it and hitting the range.
I think that once I get my .30 cal suppressor ordered and recieved, there will probably be a 300 blk in my future, so that might well be my next AR-15 build.