I read constantly about upgrading the performance of 5.56 platforms, but nothing about something similar for 7.62 platforms.

Is there no room for improvement, not enough room, not much you can do within that cartridge-length envelope, or is 7.62 considered "good enough"?

Even the .50 BMG has the .416 Barrett...

6.5 creedmore maybe

Could be that 7.62/.30cal. is typically overkill on a human.  5.56/.223cal. is more of a varmint round, so bullet construction is of more importance when you start to get into deer/human type targets.  Most folks looking at the upgraded calibers are trying to get more terminal performance with less recoil than the 7.62/.30cal.  I really like the 7.62 round.  Great energy to target, I can control the recoil (not talking full auto capable here),  and accurate.  Of course, like most things, the round/platform depends on what your purpose is (thus the .416 Barrett).  HTH.

Many variations of the .308 parent case are around and any of them can be used in the .308 AR platform.  .243 Winchester, .260 Remington, 7mm-08, .358 Winchester, to name just a few.  There are also several other rounds not based on the .308 that can be used.  The 6.5 Creedmore has also been mentioned, there is .300 RSAUM, and many others.  I think you don't hear about them much because there isn't much a .308 can't do in a battle rifle.  It wouldn't be my first choice hunting large dangerous game but for humans it works quite well.

IMHO the only think a .308 caliber battle rifle needs is better bullet selection than the crappy 147 grain ball load.  Say a 155 or 165 grain Matchking for that extra bit of "reach out and touch someone".

There are some folks who are trying to put .300 RCMs and .338 RCMs into .308-based uppers.  TeppoJustu.com also has the Rhino family of cartridges.

Best,
JBR

or 155 amax