Jewels. These were awesome and much more interesting than normal gems.
Legendary gems, levelable or static is fine.
More item enhancing consumables like rammalammadingdong
Open trade of anything you find
Private loot (balanced nicely between self found and trade economy, no D3 raining legos).
Eight man parties (or more)
Strength, Dexterity, Vitality, and Magic/Energy stats and allocation but implemented much better than in D2 and not at all like D3.
A pace of combat somewhere between Diablo 1 and 2
Kanai’s cube, but that is just how you equip legendary powers. They are no longer found on items.
Uniques instead of legendaries. Let legendary powers compete with other legendary powers. Let uniques and rares duke it out on the merits of their affixes. Legendary powers appliable to rares means rares are BiS, just need to find good ones. Bad move. Rares and Uniques should be viable in any slot, depending on build and affixes that can roll for that slot.
Useful crafting (cube or otherwise)
Diablo 1 atmosphere and music. Bring back kight radius. Make the world darker outisde of that radius when appropriate. In D2 overworld was lit, but dungeons were dark except light radius and other light sources. If there will be day/night cycle this will be better.
Limited respec exactly like D2.
A type of content like normal Rifts - completely randomly generated, no timer. They would be something to do if you got bored of the dungeons or overworld farming. Perhaps they could be endless dungeons and every floor got harder and harder. If you die or exit, it ends. Sort of a guantlet, untimed. Who knows though, maybe dungeons and their keys will scratch this itch completely.
Magic Find as an affix, averaged across party for group content.
Mercenaries like in D2, but more equip slots (same as player) and D3 talent trees for them, but even better. Active in group play like in D2.
Monster resistances! I am okay with monsters not having immunities, but definitely give them resistances, and have elite affixes grant resistances like in D2. Perhaps base monster type being resistant to fire and then having an elite version that is fire enchanted should be the only scenario where immunities can exist.
If you’re going to give every class its own special resource, dont make end game builds all about finding ways to not have to think about resource. If you have them, commit to the design choice, or don’t have them.
If you’re going to have cooldowns, dont make endgame builds all about never having to wait for cooldown. If you have them, commit to the design choice, or don’t have them.
Paragon, infinite, but with approproate mathematical limits for each affix (meaning DR and you’ll never hit the limit), no attack/defense, and far, far less paragon levels attainable in the scope of a season. Maybe 100 on the high end, with 10-15 points before an affix becomes DRed heavily. Add tons of options for affixes, so every build has something they like, and its not all cookie cutter across classes.
And most importantly… Necromancer.