7.62 is really the wrong answer to just about any problem the military has these days. Like the 9mm, the only reason it is around is due to legacy issues. There are reasons those cartridges have been dumped in various times and places. We don't need to move backwards, we need to move forward.
6.5 mm is the way to go. Longer effective range than the 5.56, flatter shooting, and the bullets are a bunch heavier. This answers all of the concerns that the folks in combat haved raised (besides the lube issue, and that is independent of what rifle our forces use). I don't think that it would be too hard to put a 6.5 mm upper on an M-16 lower, but that doesn't much matter - for what Uncle pays for rifles (a couple of hundred bucks), I say to just buy brand-spanking-new rifles and mags. Since all soldiers are familiar with handling and sighting in on M-16 variants, keep the same look - but beef up the caliber. If I were in charge, I would put the M-4's sighting system on all of these new rifles.
And .45 is the way to go in combat, since you are restricted to FMJ by treaty. Hollowpoint 9's are fine, but the military can't use them. And if you could use hollowpoints, why wouldn't you want to use a .45 cal. "flying ashtray" anyway?
You are probably better off with a .40, and some modern non-NATO militaries have in fact adopted it (Australia comes to mind). Most of the cartridge capacity and ballistic performance of a 9mm, a fair portion of the size and weight of a .45, and more down-range energy than either 9mm or .45. The .40 splits the middle pretty damn well for a human-killing cartridge.
That we use either .45 or 9mm at all is primarily a legacy of previous wars, and the engineering assumptions that went into both of those cartridges is a century old. One thing to remember about the .45 was that it was originally marketed as a ranch pistol for plugging critters before it was adopted by the US military. There are certain biases evident in the design of both the cartridge and the pistol as to its original intended purpose that make it sub-optimal as a combat cartridge. The 9mm has better engineering parameters for the purposes of a combat cartridge, but nonetheless suffers from being a bit smaller than what is probably optimal for spanking determined monkeys.