Having Cumberbatch is fine. Having him as a villain is fine. Having him play John Harrison is fine. Having him play Khan Noonien Singh is just silly.
This. I'm perfectly fine with The Batch as John Harrison or any other new villain, but having him turn out to be Khan is just a wrong fit. I mean, I thought the whole idea of creating a branch universe in the first movie was so they could tell stories that are not strapped by years of Trek history and details. So why NOT create a new antogonist?