LSU isn't going to leave the conference they have 100 years of history with for less money, exposure, and stability. Plus, they have recently been a national contender in that conference and have proven they don't have to be a perennial also-ran like Arkansas or A&M. It's an extremely small chance, but I could see a scenario where Arkansas could be convinced. As was said, they went from a national brand to regional also-ran in the SEC. Their fans still love to hate Texas and would enjoy the rivalry as well as playing OU and other former SWC schools. The biggest thing they'd lose is the money, but we could close the gap with a school of Arkansas's value. They left the SWC for the same reasons we did. Their administration wasn't as petty as A&M's about going to the SEC.
But the biggest hurdle I see in trying to convince any other P5 school to leave their conference for the Big 12 is the reputation of instability this conference has or that it may all come crashing down when the GOR ends. We'd have a better chance trying to poach PAC schools who may not enjoy being last in revenue or some ACC schools who haven't been there a long time or have much history with the conference.
My "never going to happen but a guy can dream" scenario would be Nebraska, CU, and Arkansas all feeling regret about their diminished prestige and lack of historical rivals in their new homes and agreeing to come back to their Big 8/SWC roots, putting us at 13 with only one spot to fill.