Some ideas from some random dude that played Diablo 2 LOD for 20+ years.
QoL:
Increase the amount of storage space, either account shared tabs or increase the amount of character slots. Priority #1!
Crack down hard on exploits, bots, and hacks. I mean multiple ban waves each ladder season. Or maybe let them do exploits, bots, and hacks in a server separate from ladder and non ladder, Open Battle.net; then not allow those who use those exploits, bots, and/or hacks to create Ladder characters.
A character stash preview inside the character selection screen would be a huge QoL to save time from searching each character.
Self-Found Mode, cannot join or have anyone else join your game. You cannot use players x command and add a leaderboard.
Stackable items such as gems, runes, essences, keys, tokens, organs, etc. This will aid in increasing item storage.
Increase the crafting items variability like chance to cast cleanse when hit, rolling resistances, chance to cast frozen armor, chance to deadly strike, chance to freeze, etc.
For PVP:
When PTR hits, this has to get fixed. Listen to the community with the focus of bringing balance and sticking to the roots of skill and timing.
Easy fix: Fix the FHR and FBR back to normal. Pre 2.4
Add a variable cooldown depending on the character’s FHR activating when 3+ consecutive stuns occur in a short period of time in PVP to remove stun lock.
This will remove stun locking but still preserve the essence of stuns.
Utilize the PTR on this, make sure it focuses on removing stun lock but still keeping the value of properly timed stuns. It will most likely be a fine tuning of how long the short period of time is: 1-5 seconds.
Other Ideas:
Add more runewords for class items that don’t have runewords like sorceress wands, necromancer shields, barbarian helms, etc.
Add synergies for frozen orb either increased chance to really freeze enemies (so blizzard can hit a non moving target) or % increased damage. It is and always will be my favorite spell but it is weak as an end of spell tree skill.
Season Theme:
A possible season theme could be crafting charms required to collect crafting materials from a random Uber spawn with a 50% chance to spawn in a game in a random area some where throughout Act 1 - 5 to increase every areas play time. The crafted charms would have a higher chance of rolling a prefix and suffix stat with a unique stat.
Add a new Survival mode, Endless Cows. It is a cube recipe where you have to collect rare materials that drop from the Cow King in Hell. It creates a cow horn that you can use one time in Hell Tristram, it blows a sound through the horn you can hear in game. It turns Tristram into a survival event where portals spawn around Tristram that spawn endless waves of cows. The Elite Cows progressively increasing monster level as you kill the elite/unique cows but the common cows keep spawning at the original monster level in hell. Once you get to monster level 90+, the elite cows start dropping 3 items. It adds skill, a leaderboard, good way to get exp and drops, end game, replayable.