5.56 is going to be around a long time. The recent extension of Army infantry school and the inclusion of more live fire training, along with enablers like NODS and lasers will make a bigger difference than any change in caliber.
The cuwrrent push for an intermediate caliber is based on 1) longer range engagements in Afghanistan and 2) the idea of pushing through body armor plates. 6.8 is inferior to 5.56, particularly the M855A1, for both of these tasks.
Farnham needs to avoid the trap Col. Cooper fell into wherein he idealized the past and spent a lot of time re-fighting the boer war in his head. Re-fighting the last war is a pretty common trap.
Barring something truly innovative, the idea of an intermediate sized, “AR 12.5” to go with the intermediate cartridge is worth a look. The Army is already using the new “small frame” DPMS GII AR-10s to experiment with intermediate cartridges like the .264 and .277 USA.