It's already available in .264 - the 6.5 Grendel. Works great in standard uppers with up to 144 gr. bullets.
And the 6mmAR. It wouldn't be a stretch to go to 284. We've already had .277 in the 6.8SPC.
The 6.5 Grendel is based on the PPC case which in turn is based on the 7.62x39. The rim diameter is .445" which is about the limit of opening up a standard AR boltface.
The 6.8SPC's rim diameter is .422"
The new 30RAR is based on a .473 rim (same as the 30-06 and 308, which is too big to fit a standard AR bolthead. The new Remington cartridge is essentially a 30BR (6mm BR opened up to 30cal). As others have said, it's got significantly more capacity than the Grendel.
