I think South Carolina is keeping its forced cross-division rivalry with Arkansas, so that TAMU and Missouri can play every year. If it were up to me, I'd switch it, so that Missouri and Arkansas would play annually.
Regarding rivals for Missouri, I think Arkansas would be a great possibility, with the states bordering all the way across, but the football schedule isn't making it an annual game. Kentucky is another possibility. It's the two northern-most schools in the league, and they're both good at basketball. Gazing at a map, it looks like Missouri's small border with Tennessee is a tad longer than its border with Kentucky, but the Vols already have several established football rivalries. If Mizzou's football program slows down amidst the weekly SEC gauntlet, they may find themselves in a rivalry with the Cats.