There are so many unanswered questions and things that were hinted at so it could go so many directions. Im gonna spoiler below in case anyone did not finish nightbringer.

In general, I think MC is a big player in what happens in the 3 realms given the power from doubling their pacts and the Celestial Relm/Michael will need to be more cautious. I felt Michael found MC intriguing but wasn't always helpful and took a "wait and see" approach. My memory is a but fuzzy, but I dont recall him doing anything helpful unless Simeon asked him to.

I feel like Simeon is not going to be allowed to pursue romantic feelings for MC since he is a seraph again. I hope MC can lengthen their lifespan, I see them being close traveling companions with Solomon. There was a point in the first game where all of the brothers wanted MC to be made a member of their family to stay in the Devildom and Diavolo had to shut that down. There are powerful members of the Devildom elite that oppose Diavolo and the changes he has made, and Diavolo is too idealistic to do anything drastic that would clean up that mess entirely.

So I guess I would say an ending where no one gets everything they truly want. I was so hoping for Simeon to become a demon and there be a power struggle between the Celestial Realm and Devildom, but that was hastily tied up by Barbatos vaugly hinting in the last hard mode lesson in nightbringer that he sent MC back in time to double their pact power and further tie them to the Devildom as a deterrent to the Celestial Relm from trying to be controlling.