Think about it: we're practically guaranteed to get a He-Man and a Skeletor variant per year, which leaves two slots open. The only others that had variants in the vintage lines were:
Granted, there were the Snakemen variants in the 200x line (Stratos and Mekaneck, plus MAA and Orko) and the later NA variants, but I can't see any of them selling well. Others that seem like popular suggestions are:
King Randor (Filmation)
King/Captain Randor (200x)
Teenage Prince Adam
Add to that maybe a 200x King Hiss and Marlena in a white Rainbow Explorer costume...
So I think it's quite likely we'll see a POP variant of She-Ra or Catra some time soon, but I'd rather they focus on other members of the Great Rebellion first.