As much as I loved wow I must say after 8 years or so it's about time with something new and fresh from blizzard. You need something that is challenging, because we all saw how it went with D3, it was so much people who played it at the start but once they nerfed and people cleared things super easy and fast it quickly faltered.
It's always the journey to the goal that is fun, you saw that specially in Gw2 and d3, it was so frigging fun to level up, I tried doing rakanoth with my badly geared DH with no defense in inferno, hell I even struggled abit on hell, that was fun and I felt really good after I beat him.
We need a progression system that isn't forced but necessary if you want the best gear, coolest mounts, coolest titles etc, I'd personally would love a system that allowed me to progress toward getting access to a new map or new dungeon, instead of getting everything handed out or having gear requirements, if it's available to all it will mean nothing to those in it.
To stay on topic lets say it's a D3 spin off. As a progression system you could have a spell upgrade system, so you ding lets say 50, you get a set of regular abilities, but you can do tasks, quests, dungeons, search for rare hidden chests etc etc with upgrades to those spells, like as a ranger/archer etc you could start of with a single shot ability and with some hard work and dedication you'd be able to power up that ability to shoot 3 shots that spreads if there multiple mobs or fire 3 at the same guy if it's only 1 for example.
The standard mmo nowdays for most people, is basicly hurry up to max lvl, get the best gear and wait for expansion and if you're bored there is tons of achievements you can do but neither gives you a real benefit and nobody really cares that you killed 50k critters or have 250k honorable kills.