I don't see the need for Mephisto in this setting. They just revealed a cross dimensional gate and a big bad witch, not sure we need some dude behind it.
My guess is, if Mephisto or Nightmare or whatever is really behind all this, they won't be a physical presence in the show. Just a lingering threat for Wanda and Strange to clean up in Strange 2: Electric Magealoo. We've got our big bad reveal with two episodes to go, trying to cram in ANOTHER one outside of a post-credits scene would feel really rushed.