Uhhh, I can think of one big baddie they haven't faced yet, but he's literally big lol that one planet-eater Galactus.
Most I've seen of this guy was through a Fortnite event lmao don't bully me
Idk anything about him other than the fact that he's a dude in the universe who consumes planet energy and that he's somewhat related to Silver Surfer (idk if they're partners or enemies, but they have something to do with each other). He seems like an Avengers-level threat, it'd be cool to see him in the MCU
EDIT: Oooh hold up, I remembered something. Idk where I got it from, but Silver Surfer gives me the vibes of a messenger warning the Earth of Galactus' arrival. He's very neutral towards conflicts, I think. Of course, I could be wrong, but like I said I don't know anything lol at least not with certainty