The World Quest system and all the Diablo-like things in Legion made me wonder if one important thing from Diablo 3 should also be added now.
WoW character progress is 100% exclusively via gear. If you don't get better gear, it's useless. That's how WoW works (unfortunally). It's a bit one dimensional and also limiting the rewarding system (giving mythic or heroic loot for clicking something on a table always felt wrong and exessive).
We need smaller but still meaningful endgame rewards outside of raids to make people play in the world even when their gear is already BiS. That would be good for the game if you meet players all the time in the world and not just at the start of the expansion.
Imagine you could continue to level "Paragon" levels doing outworld content like the new World Quests.
They could increase other things like the speed how fast you can gather, craft or cook, more gold dropping from mobs or a small movement speed boost (let's say capped at 10%). Just like Paragon levels you would need to spend points in those, too to continue spending in base stats. When you capped all stats you would only increase your base stats. The great thing about this is, you don't need a group (but they could give a small paragon xp boost for playing in a group), you can do it at your own pace, everybody gets it and if you want more, you just play more.
The final difference (at the end of the expansion for example) should be measurable there but not gamebreaking high (lt's say the dps difference between a high paragon level player and low paragon player with identical gear and skill is 5% max).