I'm not sure what d4 needs, but I know things I liked and didnt like in other diablo games.
In d2 I liked the rune system. It felt rewarding to collect set of them and then use them to make items badass.
In d3 it feels like majority of skills and their runes are useless. Why are there so many uninteresting or bad skills?
Also, d3 talent system is bad. It kills certain type of replayability, but not sure, if its good or bad. Lots of people might just stop playing when they find they made their character wrong.
In d3 (most?) sets tie you to specific skills. I feel like sets should not do that.
Just some things that came to my mind quickly.
Edit:
Also... d3 classes were boring. This might tie to the earlier point of majority of skills being boring.
Where was druid? Where was necromancer(though, necro came later, but in my opinion it suffered from same problems as rest of the classes)?
Honestly, d3 often felt like it lacked... kick. Yet, not sure how much better it was in d2 (for its time).