Different activities tend to have different reasons for doing them. Nightmare dungeons power up glyphs whereas other things tie into boss encounters and “mats” for a higher chance of getting the best items. One of the problems this game has, particularly in the open world, is a grind coupled with a lack of challenge so mind-numbing that playing the game becomes revolting. The Duriel-centered loop that constitutes much of the endgame is downright awful when you run out of accumulated mats and have nothing else to do. So why not take an open world activity that spans the entire map and has its own somewhat varied quests - the Tree of Whispers - and turn it into an actually legitimate endgame activity?
You collect 10 Grim Favors and when you go to the Tree you have the same collections of rewards with one additional option. The additional option is to turn in the Grim Favors for an upgrade of the Whispers of the Dead zones. The zones become more dangerous (stronger monsters and elites, greater density, boss encounters, etc.) but the rewards become more compelling when you next visit the Tree with 10 Grim Favors. The choice to upgrade zones repeats. You can be limited to 4 lives in the zones once you’ve made upgrade bargains with the Tree; you either collect 10 Grim Favors again or you die enough times in the zones and lose out on any rewards so you have to be considerate. This activity should either be appropriately controlled for parties and multiple players or it should be restricted to solo play - give us loners some love.
This could go up to a point where you have an alternative open world activity for uber uniques, say at half the percent chance of a successful uber Duriel encounter? Maybe it could give you extra paragon points beyond the cap at level 100 so that you can go for more glyphs or nodes?