My problem are 2 things (well, a bit more than 2 :P)
1 - Post level 45 the “Increased Health” starts growing too much, especially felt in the Pit runs. It’s fine if elites have a lot of HP (albeit a bit/tad too much), the problem is “regular” mobs with Increased HP… WHY ? 
2 - Post level 45 the game loses its “soul”… You no longer care if you fight bears or vampires or flies or khazra - they’re all the same… Actually the only difference is Fallen and that difference comes from Fallen Shamans resurecting stuff all the time… I know people don’t want to hear this but - the mobs are too harmless now
3 - Never before have I had runs in the Pit where 4 minutes is NOT ENOUGH to kill the boss… Like, 4 minutes 1v1 vs a Boss that can’t even pierce barrier yet alone HP and still - woh woh woh wooooh with the Timer, like, come on
SO ?, the solution:
1 - Buff monsters back, not damage necessarily but at least give a few of them some CC, make it different to when we fight knights, vampires, bears, e.t.c., make that difference be felt again
2 - Reduce scaling, remove or at least rework “Increased Health” augmentations of individual mobs, try replace with something different and more interesting for a change
3 - Stop scaling at a certain point, say that be at pit 80, add more and more of those augmentations instead
4 - Nerf the heck out of DPS, nothing should do more than 7 figures… It’s gonna be harder to balance unless the “range of operation” becomes within 1 digits of a factor, let strategy take over, not scaling or DPS increase beat it all
5 - Experiment some more, open pit from lvl30 or 35 and rework the Paragon to make it possible to “hide” some weakness of a build, for example Stun duration reduced, or Poison duration reduced, e.t.c.
In other words and TLDR ? - make the game work, not allow the scaling or DPS increases completely take over, let the game, work. Heck, consider even having 2 paragon boards, one with all offensive nodes and one with all defensive ones 
And then nerf boss HP some more to more comparable numbers, don’t make the game completely worthless and requiring one-shots except boss fights which can take a lot longer