For me: Making it feel like an 'adventure' again, treat the players as adventurers, rather than being a champion or leader of something just doing their job to save the world. It would also focus on the class fantasy the right way (since blizzard admitted, they went for spec fantasy too much this expansion). Make things feel like they would belong in the same world as Warcraft 3 (since I feel it's gone way too far from that). Give us villains that are more relatable, something more similar to us, rather than all these things we get nowadays. Though granted, having a new Nefarian like villain would be fun, kinda miss having someone not TOTALLY breaking the 4th wall but he knows what's going on.
No artifacts of any sort, make Legendary stuff something you get from raids. Also Guild Halls would be pretty sweet - each faction can pick a 'theme' from the races, in case someone would like the typical spikey orc stuff, or the magical blood elven stuff.
More talents would be really nice too after the whole leveling change. It's just boring at lower levels when you take a while to kill stuff but you got so few abilities. Have a talent for every 5 or, if needed, every 10 level.