Although theoretically and metaphysically possible, I doubt there is a lot they haven't tried. New things do come along (they're using 90 grain 223 bullets in Palma matches these days) and advances are made but the physical limitations are still present. Consider most of the short magnum cartridges or even the Weatherby Magnum cartridges. They're a few hundred feet faster with a little more energy but eventually they run into the upper edge of the ballistic envelope also.

As an interesting side note, have you considered just bumping up to a 243 or 6mm cartridge? Your deer ranges (and killing ability) would increase substantially and you would still retain the ability to whack prairie dogs just about as far as you can see them.