The only way either would ever become popular is if NATO dropped the 5.56. The reason .223 and .308 are everywhere is because both are main cartridges in NATO's arsenal. Surplus ammo used to be widely available. Given the nearly fifty years of the 5.56 use, and the nearly sixty years of .308 use, plenty of rifles were developed for them on the civilian market. Hence why so many non-contracted ammunition manufacturers started making their own versions of the .223 and .308.
Now however, the world if a different place. Everywhere you look governments and scared, delusional anti-gun parties have sprung up with this radical idea that by infringing on the rights of law-abiding citizens, your protecting them. Its strange, they're logic is along the lines of "if its dangerous, ban it". Heck, Britain recently started confiscating certain bladed items, weapon or otherwise, and even created the term "assault knife".
For a new round to come onto the marketplace would take a drastic change in the world. NATO would have to have a crisis that demanded improved ballistics over the 5.56. Then, governments around the world would have to use the brains they were born with and realize that the only way to fight crime is just that, FIGHT it. Don't ban the gun that the drug-cartel used, fight the cartel! Do not restrict the rights of sane, law-abiding citizens, instead place laws that restrict criminally insane persons and unlawful persons! If all this happened, you'd see the 6.5 or 6.8, cheap and plentiful.
But, we live in a world where people prefer to just watch their MTV and VH1 and pretend that nothing is wrong. Then when something does go wrong and MTV breaks for its 15 minute news, all these half-wits who half-paid attention only hear "gun" and "death". They don't listen or care about anything else, only what they saw in their tunnel vision.
There's a local guy who's been trying to sell his Baby Eagle with the 9mm and 41AE barrels locally for several years now. So far - no takers. Go figure....
