if I had my choice, I"d make a serious run at Pitt or Iowa State. Nebraska's not leaving the Big 12, Missou would be another interesting choice, but I think the tie to Kansas kinda hurts that.
Pitt's big rival is supposedly WV, but they have Penn State as previously mentioned there, plus, it's WVA, I think there's probably 4 non fans of either program that can even tell me the name of their rivalry. (The Backyard Brawl LOL, WVA is nowhere close to Pitt's backyard) Wannstedt has brought them back to where they're at least relevant contenders in the Big East, they'd be no worse off at the start than Indiana, Illinois, and Michigan are right now. (Can't believe I just said that.) Plus Pitt makes the Big Ten MUCH more relevant than they already are in hoops. Pitt/MSU twice a year would be heaven.
One that's not really getting any mention is one I'd love to see. Iowa State!! The Big 12 won't miss them, they'd have no more travel expense than Iowa does every year, so geographically they fit, Iowa/Iowa State all of a sudden becomes both a football and basketball rivalry, and I think ISU's football recruiting actually would get a boost from being in the Big Ten, oddly enough. Recruits know you're not competing for a conference title with Texas, Oklahoma, and Nebraska in your conference. The Big Ten's only real mainstays right now are Ohio State, Penn State, and Wisconsin(Iowa spends half their time here, the other half, who knows). The midcard of the Big Ten usually sees 1 team pop up for a year or three, then drop back down, but they could compete with all of them. Some may disagree, but I think it makes sense both for the program as a whole. Probably not even an option, but just spitballing I guess LOL.