Well, what we need now is a qualitative jump in the barrel metallurgy. Barrel metal is the bottleneck. If, say, barrel resistance to abrasion and gas erosion could be jacked up, then we might get much better rounds than 30-06, Weatherbies or Lazzeronies.
Why use metals at all? I propose a carbon-fiber barrel with a ceramic liner.
Barrel durability is not a problem whatsoever in small arms. Building a barrel that will last nearly forever under heavy use is not an attainable goal with current materials. But building a barrel that will last long enough to perform any reasonable shooting task and then be replaced is well understood and has been for decades.
By far the most demanding application for gun barrel technology is in front of a revolver breech air to air cannon as installed on fighter aircraft. These fire 25 to 30 shots per second with chamber pressures in the 65000 PSI range. They will wear the rifling out of a barrel in just a couple of seconds. But as the aircraft carry only enough ammunition for a few bursts, that is quite good enough. The barrel is changed after a mission or two in which it was actually used, and that's that.
A barrel is a consumable, just like a drill bit or a saw blade.