Don't bag on Iowa State so much. They are an outstanding research institution. Yes they would be a bottom feeder in some sports. But their facilities are good, and when they have had the coaches they win. The currently have the best Wrestling program in the Big 12.
Also keep in mind that all the current members of the Big Ten are members of the Association of American Universities. Which is a prestigious group of 60 research universities.
The new member would likely have to come from that group.