I wouldn't bet on it being implemented, the US army has a very long track record of looking for the "perfect gun" only to nuke the whole thing at the last second. SALVO, SPIW, OICW, ACR, the introduction of the SCAR turned out to be mostly homeopathic.
The US military has been obsessed with a single trigger pull rifleman's weapon since the 19th century, they passed up on repeating rifles in favour of the trapdoor Springfield. They did keep pace with other nations introducing repeating rifles like the Krag Jorgensen and the 03 Springfield and even briefly outpaced everyone else with the M1 Garand only to introduce the M14, designed to replace not only the M1, but also the M3 SMG and the BAR.
The M14 was specifically designed as a single shot rifle first and an automatic weapon last, going for a full-size cartridge, though the Germans and Soviets had demonstrated that an intermediate cartridge was the way to go, something the British also were investing in, but were told that nothing short of a high-octane, red meat eating man's bullet, capable of blowing a ed commie in half two miles away with a single shot would do for NATO.
And because of this obsession with full power ammo they started to work on a better rifle before the first GI even held an M14. SALVO was a double-barrelled rifle that fired duplex rounds, so that each single trigger pull unleashed four projectiles. SALVO begat SPIW which begat the ACR which begat the computerized mini-fridge size OICW and the search goes on …
The M16 was the only time the military allowed designers to sneak in an automatic weapon for the rifleman, and the M16A2 removed the automatic feature in favour of a three-round burst, returning to the idea of a single bullet or single trigger pull being the only acceptable way for a soldier to fire their rifle.
But doesn't 6.8mm ammo bring a bunch of advantages over the 5.56mm, long accused of being nothing more than a glorified "varmint round" ?
Sure, it's a round with excellent ballistics, certainly better than the 6.8mm and gives your rifleman a greater range and hitting power it's not on the same level as the 7.62 used in machineguns and long range rifles. So you still need the 7.62mm for proper ranged fire and barrier penetration.
It also involves the cost of introducing at the very minimum new uppers, or even entire new rifles, hot on the heels of a new improved M4 carbine.
The new ammo will weigh more, not a good prospect for troops already expected to trick out their rifles like Christmas trees so the new 6.8mm is probably going to kick harder.
The advantage you would get from the 6.8mm is quite minimal, all things considered. I think the military will hold out until somebody comes up with new tech that offers a significant advantage that warrants stepping away from the 5.56/7.62 binome. The 6.x option was visited on several new rifles like the SCAR or the H&K 416/M17 and some other proposed rifles, but never retained.