Here are my own opinions and ideas, I know some are not realistic but I just put it here to share my thoughts.
1. Remove 300% base mf cap.
2. Add in weapon switch hot key just like d2's (mf swapping, weapon swapping for different passives skill bonus)
3. Unlimited paragon levels.
4. Remove entire RMAH.
5. Add in Cash shop to buy character skins or equipment skins ( more character customization), double mf, double gold find, double exp coupons.
6. Monthly subscription fee (I dont know if its a good idea, but as a developer, it should be good for the company since rmah is removed)
7.Hold special events once in a while for players to get unique event items.
8. Double exp event for a short period of time on every weekends.
9. Remove all level requirements on all skill runes (runes only, not skill itself), make runes selection irreversible, If you want to change to a different rune, use the cash shop or make a new character. It will encourage players to play the game more and longer to try out different builds out of curiosity,
10. Bring back the elemental effects on weapons and skills from d2. (not just cold/snare only)
This post is not up for any serious debates, Im just sharing my own opinions, you can add in whatever you can think of as well.