I think most of us would like to add Notre Dame if the conference was going to expand to 12, but we all know that Notre Dame will never join a conference as long as they have their own TV contract. We have also heard other teams thrown around such as Pitt and Missouri. I really don't know why either of those schools would leave their current conference for the Big 10. Pitt would be helped for football, but would probably be taking steps backwards in basketball, not to mention that they would lose some exposure to big markets such as Philly, New York, Boston, and D.C. by exiting the Big East. As for Missouri, I have no idea why they would want to move away from tradition, history, and rivalries of the Big XII.
Why not look west for other schools? When the conference took Penn State, that seemed pretty far east to me but it works today. So why not look to expand the footprint of the conference even more? Here are some smaller schools that I think could be deserving of a promotion:
Boise State: They have proved they belong with the big boys year after year. They would certainly add a different element (think hybrid Pac10/Big 10 with a lot of creativity). They would probably become less gimmicky with a promotion to the Big 10 as they get better recruits. I personally would love to see a game on the smurf turf some time. Their basketball team has been okay, but would definitely benefit from an upgrade in recruiting if admitted to the Big 10. Their wrestling team could also give Minnesota and Iowa some competition as well since BSU currently competes in the Pac10 for that sport.
TCU: The Horned Frogs actually have an outside chance at the national championship game this year if they win out (Alabama loses the Iron Bowl and then beats Florida in the SEC Championship game, or if Texas loses a game). Plus, this would be a strategic move for the Big 10 to establish a bigger presence in Texas for recruiting. TCU has finished in the top 25 three of the past four years and this could very well be the third time in five years that they have finished in the top 11. The problem is that their basketball team blows, although a promotion to the Big 10 would help them some.
Utah: The Utes have built a strong football program over the last 5 years. They arguably were a national championship contender last year and have only a loss to Oregon as a blemish on their record this year. They travel to Fort Worth to take on TCU next weekend which should decide the Mountain West champion. The Utes reached the NCAA basketball tournament last year after being absent in recent years. They do have a history of having good teams and could compete with the rest of the Big 10 right now in either sport.
Air Force: The Falcons always seem to have a respectable football team. I personally don't understand the hatred for the triple option since I think it is fun to watch the deception, but some people seem to have a problem with it. Their team always plays hard, is never outhustled, and is always in the game. A switch to the Big 10 probably wouldn't have much, if any, change on their recruiting (but it could positively affect the quality of their coaching staff) given that it is a service academy. Air Force usually fields a decent basketball team, as well, although they were terrible last year.
These selections are based on athletics alone. I don't know enough about each institution's academic record to speak intelligibly about that consideration. Facilities would also be a factor, but I would think that any of these schools, with the exception of Air Force, would be willing to expand given the growth of revenue sources from the Big 10 conference.