With the recent death of my favorite Marvel artist (Bernie Wrightson, RIP!) I’d love to see Tordenkakerlakk from my favorite comic as a child (Hooky).
The boss would, as mentioned, be permanent with random progression rewards.
He would not be beatable, however resettable at any time for new progression rewards;
You basically play against him for as long as you can endure and get some stuff at certain points of progression. His health goes from 0 - infinite and there could be new rewards at 100, 200, 500, 1000, 2000, 5000, 10000, 15000, 20000, 30000 etc - the higher damage the better rewards of course.
Once he downs you, you can have another go at him where you left off, or reset him to zero again.
Wow, imagine this immortal Boss fighting with your whole roster (you start with the weakest, that means lowest level, characters and when someone dies, another one appears).
We would finaly have the reason to actually build full roster with 150+ unique covers (or how many of them are there). Tho i dont think it would be a good idea to make him resetable - i would make it as a new Daily Quest