spoiler alert: i linked this
Unless there is a football power that is available (as Penn State was in the early 90s), the most salient selection criteria should be something along the lines of "institutional similarity" to the schools in the Big Ten. For the most part, the Big Ten is comprised of flagship state schools with very good to excellent academics, research prowess, and well-rounded athletic prowess, in no particular order (were I to pick an order, I would say the first two criteria are prerequisites, but that's just my particular bent). Extra considerations are rivalries with league schools, proximity, and the catchall category of "soft variables."
ACADEMICS AND RESEARCH PROWESS
Colorado is no slouch. It was ranked #77 by U.S. News in national universities, and has strong graduate programs in law, engineering, geology, physics, and education. Additionally, it is affiliated with the AAU.
The football team was a national power in the 1990s, and remains a solid program, with a large national alumni base, and better national brand recognition than any of the schools under serious consideration (with the exception of Nebraska, which is not really under serious consideration).
RIVALRIES WITH LEAGUE SCHOOLS
Michigan fans of a certain vintage break out into a cold sweat at the very mention of the names Kordell Stewart and Micheal Westbrook, and the (in)glorious 64 yard hail mary. The 1994 game was one is a series of good games, including 1996, when Michigan got revenge at Boulder, and in 1997 when Michigan steamrolled Colorado, making it clear the '97 team was special.
There is admittedly not much beyond that, other than a Wisconsin-Colorado bowl game from the early 00s. But other than PSU-Pitt, no other team under consideration offers the rivalry factor, and you have to go back even further (i.e., the early 80s) to find a time when said rivalry had national implications.
Under the rubric of rivalries, it is worth mentioning that only one of Colorado's true rivals (Nebraska) are in the Big 12. They could open the season with Colorado State, as they have always done, and schedule Nebraska non-conference from time to time.
I have been to Boulder, and it feels like a Big Ten campus to me. A great college town. Also lefty, hip, and scenic. Lots of places to see live music, etc. Periodic road trips to Boulder beat trips to Syracuse or Pittsburg, for damn true.
I think that it also has the potential to raise the national profile of the Big Ten in a way few other schools can do. Neither Pitt or Syracuse has the national footprint that Colorado has. An average nationally televised conference game with Colorado has a better chance of getting national exposure than either Syracuse or Pitt (or, for that matter, Missouri).
Geographically, this is a stretch, but maybe not all that much of a stretch if Nebraska and Missouri are in contention.