Was thinking a .224 Valkyrie.
I am in a similar quandary with my next AR build. I have a few .223/5.56 calibres now, 7.62x39, and 6.8 SPC.
I am leaning towards 6.5 Grendel. Its got more punch than the .224 Valkyrie due to bullet mass. The Valkyrie will stay flatter and reach out supersonic a few more hundred yards, but with less overall energy. Since I have other rifles for reaching out past 1000 yards, I think the Grendel will come first.
I think the 6.5 ammo is also a little cheaper.