Hear me out. Like the title says. I think Momentus is a big dude who should be able to visit anywhere he pleases. I don't know about you but I'd love to try and battle him in the Nether. Lava all around other Nether beings joining in the fight. Sounds like a better battle to be fought.
Momentus is an overworld boss. Trying to fight him in the nether would be very hard due to the lava and heights. Sorry, but -1 here.
I would think a marlix would make more sense. Momentus would take so much environmental damage all the time from lava it would become frustrating. He would die, you would take damage and get nothing, or worse your reward would drop in the lava . A marlix would at least fly above the lava reducing the chance of environmental damage but you still might loose your drop in the lava . It would be frustrating as well, I know I have been happily mining away and suddenly teleported into someone elses fight with A mini boss. It would be sad to get pulled away from where I am and put into peril of loosing everything in the lava because of a mini boss I never knew was there. I can see the attraction of it but I can also see some issues with it as well. Perhaps a certain area of the grid, say all positve x,y,z of 0,0,0 would be allowed to spawn mini boss in the nether. That way you can choose to enter the area with the mini boss in play.
I don't think it should be Momentus exactly, but maybe a pigman version or something. Either way, I still don't really want it, because of the things others have already mentioned (losing drops in lava, boss taking damage from surroundings, ect.). I don't really want a mini boss in the nether, but it could be cool.
Pigmentus* Having a giant pigman to fight would be cool. Plus, smp8 already has a statue for him to worship.
