What? No. Cofagrigus is good as it is. It really doesn't need to evolve. If it could evolve, then I'd just leave it as a Cofagrigus, slap an eviolite on it, and watch as it never faints and kills everything.
I've got to say Banette. I love it's design but it's just so bad. It's stats are pretty terrible and its movepool is worse. I know it has good attack, but it's not fast enough or bulky enough to even hit something before being taken down.