God, I hate that word, REHASH...
Don't hate on words.
Re-imagine, rehash, same difference.
Off all those, rehash is the most appropriate since it has a derogatory streak.
Bringing back another old villain would be quite pathetic for a series set in the entire Milky Way galaxy... but if they do, it's gotta be the borg. They have some name recognition among the general public, whereas Q does not.
There would be a sort of brilliant laziness to picking Q, though. You get your character and reset button in a package deal.