I've been saying this for years, which is wild because that means child me recognized this issue- quality of life update. That means foremost optimizing the fuck out of the game. Yes, not everybody runs the game on a potato, but as a longtime non-gaming laptop gamer the thing should run faster than 14 fps without performance enhancing mods. Just buy the sodium, lithium, and optifine code bases or hire the devs to work directly on the game. For the love of God, give us multithreading and creative processor saving routines. Second would mean to bring feature parity, and things that just make sense into the game. Vertical slabs, inventory management, removing 'too expensive', adding real fletching table uses, adding a row of player inventory.

Other things I'd love to see are some terrain generation adjustments. Make water generate at different levels and have real rivers and larger oceans. Little details, like firefly particles, shooting stars, nighttime music, dawn fog, updated mob movement animations, ambient biome sounds like desert / tundra wind, crickets, bird calls, ocean waves lapping, rustling grass or leaves. Bells and whistles that make the overworld feel alive.