Let us fight in a group of two to four against new buffed bosses. Just like greater rifts its on a time limit and for every 2 minutes you have left when boss rift is finished makes it up in rewards.
I think players would like this cuz I was playing a lot when game first came out and battling Belial was quite popular also Cydea was a fine boss when her spider minions could 1shot the player forcing force move. Also ressurecting was hard but doable (should I res him or keep dealing dmg?)
I know blablabla everybody hates this game so lets be nihilistic etc etc this idea would not take to much work eh? Maybe like 5 difficultys all unlocked when clearing grift 100 solo or smth like that. another idea is that when it opens. Diablo 3 counts partys toughness and give difficulty after that.
edit: could work with a paragon cap at level 400 and players distributing their points just before the battle.