The Beyonder, who is literally a living reality, would be such a villain, but his omnipotence is not what makes this amorphous being a complex character.
Not only could this make for one deliciously bizarre storyline, but it could also mean a few fun callbacks to what Grandmaster put Thor through in Thor: Ragnarok, just in a more high-concept setting.
When the Beyonder discovers Earth, he is fascinated to learn its living beings are one aspect the greater whole that makes up their universe (because he himself is one complete manifestation of a whole universe), piquing his curiosity about just how “complete” individual people are and inspiring him to snatch both heroes and villains alike and force them into gladiator-like battles with one another.