Metatron could be an interesting choice for an antagonist in "Hazbin Hotel". He's basically the commander of the entirety of God's angels and is supposed to be important enough that he stands next to God. Here's a short video explaining Metatron and why I think he would be a great addition to "Hazbin Hotel".

According to the Angelic hierarchy, Metatron should be able to one-shot Lucifer and Alastor easily. It'd be interesting to see an antagonist that can actually intimidate them and feel like a truly terrifying threat.

He looks like one of those final boss kinds of guys. He does appear to be threatening to some degree and could prove himself a formidable adversary. But the best way for him to be treated seriously is the impact he would make on the overarching series itself.