Humanity - the vast general bulk. As in, their own fears and loathing of "what is different, possibly superior" making them innately hostile and because humanity has the numbers on the planet they as a whole can do enormous damage

Posthumans - we saw one so far, and clearly in HoX/PoX these are The Big Bad. We'll have to find out how potent they become.

Sentinels/Nimrod - obvious threats usually, some call them boring because they don't really have their own agenda, but Nimrod has something of a personality at least in the future. And clearly this machine may get restarted. Karima the Omega Sentinel maybe can get put into this category.

Xeno and groups like that - spooky and intimidating, these guys are vicious and terroristic, but ultimately are they glorified thugs or something more? Maybe just the fear factor they can induce in mutants is a threat by itself.

Orchi - HIGHLY organized, effective, well resourced, with very intelligent members. An absolute threat tying into several others.

Villainous mutants - Nuff said. Mr Sinister. Selene. So on and so forth. Right now they're mostly cooperating but it can't last forever can it? Forgot to bring up Madelyne Pryor who surely will return.

Aliens - We see in New Mutants the Shi'ar and other races are definitely still around and they might get "opinions" about Krakoa and choose to do something about it. (Reviving Jean and other Phoenix Force hosts might not be looked on favorably for example)

Place your bets or add more