Lucifer-the alien baddie who crippled Professor X. If the X-books don't want to use him then I would love to see him encounter Deadpool or Squirrel Girl. He'd make a great foe for anyone.
Grotesk-perhaps he could battle the other rulers of the subterranean world like the Mole Man or Tyrannus.
Nimrod -a cross between the Sentinels and the Terminator. Has he been seen anywhere recently?
Are there any X-Men villains people would like to see again anywhere?
I also like Donald Pierce and The Reavers. The X-Men have seemed to have lost most of their classic villains (As in, they're not really villains anymore.), so it's time to either use some somewhat classic ones, or create new ones that LAST! That being said, I NEVER want to see Vulcan again!