The powers that be may prove me wrong, but I personally suspect that the first serious prospect for any change in caliber in service weapons will be if/when the LSAT project gets cased-telescoping or caseless ammo developed to an operational level.
If that happens, I'd think we'd be at a clean-slate scenario to the point where it would be possible to examine ideal caliber, velocity, muzzle energy, etc., without big picture economics and logistics dictating that "good enough is the enemy of best" and such. Presently, they seem to be looking exclusively at 5.56mm projectiles (though one of the bullet shown in the briefings on line looks like it must be around 90 grains), but there's no reason they couldn't bump the final product to 6, 6.5, 6.8, 7mm or whatever other caliber and bullet weight.