I mainly agree with BYUFan here, while also agreeing with those who think the Pac 10 is a long ways from expanding.
While Colorado (CU) and Texas (UT) may have been the Pac 10's first choice, and both rejected offers. They may not be interested later on as well. What does Texas gain by joining the Pac 10 as far as retaining rivalries. Who would their rivals be? Arizona State? USC? Stanford and Cal-Berkeley maybe? They are seem regionally unrelated other than both UT and Cal-Berkeley being refered to as public ivies. Texas fits in either a Great Plains conference or a southern conference.
Colorado (CU) maybe a better geographic fit with Pac 10 schools. I can see them being rivals with U Washington, U Oregon, Cal-Berkeley, UCLA, USC, ASU, Arizona, Wazzu, and Oregon State. They seem to fit as more natural rivals than UT.
The one exception about their image though is that they have a prairie mamal for a mascot, which fits in more with the Big 12 creatures of Jayhawks, Cyclones, Cornhuskers, Cowboys, Aggies, Red Raiders, Sooners, and especially their "Cattle Battle" counterparts, the Longhorns. Buffaloes have less of a relationship with Husky dogs, Cougars, Beavers, Ducks, Bears, Bruins, and Sun Devils.
Actually, the Buffaloes and the Ducks have had a good solid relationship for many years. While a duck isn't all that intimidating to a buffalo, a duck is often underrated and can pull tricks to befuddle the buffalo. ;) That's why Oregon's Civil War is such a chess match. Both Ducks and Beavers know how to outwit their opponents. However, it's really hard to outwit each other. ;) Anyway, back to the topic. If Oregon had to pick a permanent OOC rival, it would be Colorado. These two teams have like a "Best in the West" battle. Oregon lives to beat Colorado. Likewise, Colorado hates it when they lose to Oregon. www.jhowell.net
should have the historical records for Oregon vs Colorado. You might want to look them up sometime, sportsgeog. :)
Those are more mountain, lakes, and desert mascots. They can't find a "Cattle Battle" there that rocks the west, because they can't find some horns from a rival mamal mascot in that Cascadia, Mohave Desert landscape. But they can certainly lock horns with a loud "THUD" somewhere north of Amarillo, TX or south of Lamar, CO there in the Big 12 Great Plains that could be heard all the way to Victoria, TX to Craig, CO or to Decorah, IA.
Colorado can continue its "Cattle Battle" with Texas if both teams would agree to it. However, there's the catch right there. While Colorado would definitely agree to it, Texas might not. Texas is not known for scheduling too many strong OOC teams, especially if that OOC team would deal Texas a loss. Proof positive that Texas is afraid of scheduling good OOC teams would be Texas' refusal to continue the Arkansas series after this year. Arkansas AD was all for continuing the series. Texas was not. Need I say more on that subject? ::)
They may then approach CU and UT again, but if CU and UT are amongst their emerging new Big 12 rivals for a decade or two, would CU and UT then join. I think the split from CU's long term Big 8 rivals in the Big 12, and UT's Texas and Oklahoma rivals would be harder to split from as time goes on.
Colorado's only true rival, IMO, is Nebraska, but they are developing a rivalry with Texas as well. That's it, basically. Colorado has nothing in common with KSU, KU, ISU, etc. In fact, Colorado is the lone western outpost of the Big 12. Texas is a good fit for the Big 12 and I doubt that they would ever leave now. They have everything they need over there. Rivalries? Check. Power? Check. Money? Check. Texas might get more $$'s in the SEC, but they would lose their power and some of their cherished rivalries. Therefore, the Big 12 wins out over the SEC.