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've always wanted Benedict Kine to return in some way or fashion. Was only in a few storylines as part of the Shinobi Shaw version of the Hellfire Club.