Having old enemies return is a nice idea because it would be like seeing an old friend, as contradictory as it sounds. It's fun to see how well old faces can fit within the newer framework, so for example, Galoombas have a rather long absence, but they seem to fit in Super Mario 3D World well for basically being more durable Goombas, and Chargin' Chucks manage to fit well in the newer era for their charging abilities that work nicely in 3D.
Although, there's still room for new faces, because the Mario series is about problem solving, and new enemies are basically new problems waiting to be "solved" (at their expense, unfortunately for them). I do think this should extend to the RPG games, where it's not necessarily about original enemies but also enemies that were never in an RPG. Like for example: how would a Conkdor or Gushen behave in a Paper Mario game? That would be a way to utilise these neatly-designed enemies into a new light.
Thank you for reading.