I also wonder if the old 6mm Lee Navy round could be resurrected. With modern powder and rifles it would be really deadly. It had a reputation for accuracy too.
The only downside is the .243 would be only marginally lighter than a 7.62 since they use the same case.
I believe the perfect battle rifle cartridge was developen around 1894, the 6.5X55, which was chambered in the Norwegian Krag and Swedish Mausers.
I'd love to see the accurate flat-shooting, heavier 6.5mm round in our next infantry weapon. It would allow our people to reach out further, and to more effective at closer ranges. I'm not holding my breath - first of all, there's the NIH syndrome. Second, we have a large stock of ammo and guns that use only .223. As was demonstrated when we moved to the Garand, the expenditure of a relatively few dollars for something that would arguably be better for our forces won't be made. We'd need a shooter at the level of the Joint Chiefs or above to force that decision, which we don't have.