Harder content but with much better drops.
Rework paragon and skill trees, they are both terrible.
Do not destroy everyone’s eternal gear and stash loot. Respect the effort and time put in.
Learn from what you did right in S2.
Don’t sell half an extension where you don’t actually get to fight the advertised boss.
Buff don’t nerf.
Boss mats are a disaster. Just do something else.
More biomes that aren’t snow.
A repeatable, interesting endgame that gives continuous progression for chars (you will have to start supporting eternal for this)
Put Diablo in the game while the players are still here.
Flying sparkle pony mounts.
Maybe not the last one.