I do not believe that the game needs a full new act. What I would personally like to see is a new difficulty where the entire world is terrorized (maybe unlocking once you have a standard of heroes or just beating baal on hell.) Along with this, adding extra tiles to many of the random maps throughout the game on this terrorized difficulty. IE just making certain levels larger and providing more elite mobs to make it so a single game would last longer.
Along with this, I would also like to see more secret levels / portals. Looking at one per act. Act 1 has cows already, but act 2 -5 could use something similar. Some examples below, but they could add any twist to make them cooler levels too.
Act 2 - Arcane Prison. Just a bigger more open arcane sanctuary like map. (piece to open drops off chest for cube)
Act 3 - Flayer Village. Big flayer jungle like map with camps. (piece to open drops off gidbinn)
Act 4 - Hellfire planes. Similar to planes of despair, just large and higher density(piece drops off hellforge)
Act 5 - Arreats Shadow - Similar to any other outside act 5 map. (piece to open drops off a chest found in the frozen river once anya is saved)
With these new levels, you could also have them drop items similar to keys for another similar type of event to ubers, but developing this would come later imo.
Other quality of life features such as 2 extra stash tabs, showing passive bonuses or stacking effects in the UI, stacking mats, and newer rune words focusing on helping melee characters with clear speed / density clearing. While mosaic gets a LOT of hate, having rune words that are similar in effect for things like the zealadin, fury druid, frenzy bard, and other melee builds outside of using grief / fort would be highly appreciated.
Using Cham and Zod, in newer higher end rune woulds that add value to them would also be extremely appreciated. To add build diversity and add some much needed spice to the helmet slot in D2r, I would also like to see a runeword that has +1 to teleport in the head slot, or even make enigma be able to be made in a helmet, with drastically reduced bonus stats. I personally do not believe that enigma should be removed from the game. But I think providing other healthier options for gaining teleport without such a huge grind to first unlock would be beneficial and could spice up the meta at the start of each season.
Also, fixing the friends / chat interface to be up to date with modern design and essentially building in the traderie website into the client would be big adds imo.
Alas, these are the ramblings of a crazy man, so feel free to disregard. But I don’t think adding an entirely new expansion, unless carefully done is the answer. As D2 is a very unique game when it comes to its core systems and seeing a new expansion could overreach into mondern game design slop that makes it lose its sparkle. Please disregard grammar. 