There is not enough difference between the .270 and the .308 to squabble about. In fact you can throw the .280 and 30.06 in there too! They are all pretty close ballistically. With what you already have, you would need to go smaller, (.22/250 .243, 25.06, or .260) or larger, (.300 or .338) to make a difference.
I have not mentioned the 7MM/08, it could just be the best whitetail caliber around! It would fall in with the .270, and .308 bunch, but with less recoil!
For a real diference in your collection, a .35 Whelen or 45/70 would be nice! Good luck!