This is just an idea I had that you might like; since people are farming for legendaries by killing a boss, then quitting > reloading as it is anyways- what if you had an official Boss gauntlet that would unlock once you beat the game? I’d think there would be a designated way to start it similar to Mayhem mode.
My thought is to begin the gauntlet with whatever Mayhem mode is activated (if any) then for each boss you defeat, it adds another modifier and boosts the item find rate to encourage players to beat several bosses in succession to start getting solid loot finds. Then maybe if you beat all of them there’s a guaranteed annointed legendary or something.
I understand there may be some technical things you’d need to work out (cutscenes/maps/spawn points), but I thought you might like the idea.
I hope this helps!