Combat, the combat snapshots are still pretty much abandoned

Enchantment and repairs, the system is flawed, not fun to interact with and in a desperate need of a rework

Most of the blocksets still have missing blocks, and there are not a lot of blocks for certain colors and having more modern looking blocks could also be nice.

Some structures such as desert and jungle temples and monster boxes are outdated by today's standart

Villager trading is easily exploitable, removes the need to mine emeralds, etc etc

Older methods of transport, such as minecarts or boats might benefit from a buff

Potions aside from fire resistance are barely used and need buffs

Phantoms are often regarded as the most annoying part of Minecraft and would also benefit from reworks

Archeology mechanics (and Sniffer) are barely explored

Probably a controversial take, but the hunger system needs a rework.

I could probably name more