Rivalries feed on history, proximity, and close games.
They also require that you play one another, so I don't know why anyone would suggest Texas? That makes no sense.
We lack proximity to all the Pac-12 Schools (not that we were particularly close to any of the Big 12 schools). We don't have a history with most of the schools, beyond historical footnotes and the odd series we had before joining the league. The UU series from the turn of the 20th century into the 1960's is almost completely forgotten.
One of the largest issues is that the Pac-12 has been so consistent in membership, largely unchanged since the 1920's, and is very nicely paired, such that every team already has a rivalry or two.
Stan-Cal, USC, Notre Dame
Cal-Stanford, UCLA
USC-UCLA, Notre Dame
UCLA- USC, Cal
Oregon- OSU, UW
OSU - Oregon
UW- WSU, Oregon
WSU - UW
Arizona - ASU
All those rivalries are a century or so old, and have so much history and regional significance they are hard to ever recreate. Even the non-rivalry games have a long history.
Utah already has a deep-seeded rivalry with BYU that exists on so many levels it will be difficult to supplant. That said, if close, meaningful games follow, a rivalry can develop (UU has shown a desire to lessen the vitriolic relationship with BYU anyway). That said, I don't run into nearly as many UU alums in Colorado, shared border or not, as other Pac-12 schools or midwestern schools. Utah folks seem to stay in Utah (at least that's my experience).
I think the Arizona schools are somewhat plausible for us, as they're relatively close and may occupy a similar place on the totem pole in the south as us, realistically (on a 25-50 year view).
We will never supplant USC and UCLA's rivalry with one another, or the USC-ND rivalry. That said, if you beat a team in big games and recruit their back yard, they aren't liable to like you too much.
Developing a rivalry with a north team is tough because of scheduling.
That all said, every Pac-12 team has been kicking our *** for the entire time up to now that we have been in this league. We have a lot of scores to settle, and animosity will develop as we begin to contend. Rivalries are the largely greatest part of college football, it is something that CU has certainly missed. You can't buy history, and you can't manufacture rivalries, and for that reason, I'm happy we are playing NU in the future a few times.