One dimensional character progression proved not enough long ago, and now we get base levels, paragon, gem levels, etc. PoE went even further and there you level the Atlas as well, which I think is a good idea.
When it came to endgame content, the gameplay in D3 centered on just going up in GR levels, while PoE had multiple dimensions with Atals, Conquerors, Maven, all at the same time, while also having the mechanics of previous seasons appear from time to time. The latter often came with its own progression as well, thus allowing the player to pursue multiple goals at once or to switch from goal to goal, thus keeping the things fresher for a longer time.
Looking forward to D4, I wander what “extras” to the basic difficulty progression can Blizzard add to keep map and dungeon activity more interesting. While I don’t expect something that copies the 10 separate game mechanics from PoE, which is too much, I’d like to see more variety than in D3.