re: If you only got to keep one constant in-conference rivalry, what would it be?

This is all moot. I guarantee you that within two years of Texas joining they will be the preferred rival. Not because of the prestige of their team, but because everyone will hate them. Vanderbilt will be jostling for a shot at them. USC will be talking about dropping Clemson to play them. Alabama and Auburn fans will at last find common ground in their loathing for that school from Austin.

All of that having been said... the SEC will bend time and space to keep the Oklahoma v. Texas game going and find a way for Texas and Texas A&M to play on rivalry weekend.