PARAGON POINTS, instead of given +5 and be ONLY provided by EXP grinding, Hide those points, behind below achievements, and give instead +10 armor, 100 life per hit, 1 Yard pick up radios, +5 life per hit, X recipe, Y banner, +1 Primary stat. etc etc
Exploration: Explore whole maps, find secrets, maps, jump from cliff and fall into secret areas. Locate all chapter 1 caves, Locate all chapter 2 pylons, locate all chapter 3 graves, located all chapter 4 demon altars.
Classes: Level up 3 chars to max, have 3x barbs, get all chars to max level etc.
Completion. Complete all quest of story mode with campaign with 3 stars ( like star craft).
PVP. Unlock additional paragon by being king of the hill, win 100 free for all PVP arenas, defeat 100 friends at least once each. tripple kill with less 100 HP in a public PVP arena.
Story: Find secrets tomes, read all books in sanctuary, Locate all horadrim corpses and their journals.
Monsters: find all rare monsters in act1, kill each boss act with out gear on char, in Hell difficulty.
CO-OP and SOLO achievements…
… and More, instead of just rewarding EXP farming, we could reward all achievements with paragon points, and make them exponential, if you just focus in achievements of 1 kind the next one is less rewarding or more difficult to get, so players have to actually engage in all activities to farm paragon effectively.
Also it could be random, so for me completing the achievement of finding all rare monsters gave 1% crit change, but for another person the same achievement instead gave 1% AS. BUT not completely random at the end game we just happen to unlock different values but at the end game both unlocked a total of 30% life-steal (in 30 achievements). 1000 Life per hit ( in 10 achievements) etc etc.