1.18 : The biomes update
I feel like the "only" things that needs to be really improved are the biomes. I mean, they upgraded the end, the nether, now the underground, it's time for the surface!
More villages types ? Birch Villages ? I would love to see more animals like deers , frogs, more birds too!
Another idea : The End Final Update :
A final update to finish the end because I feel like it needs more.
To finish, these things could be updated too : The structures (strongholds, nether fortresses..), the enchantments (new villager?), more optimization (always cool), more variation of woods (birch crafting table, spruce chest...)
And there is the Combat Update Jeb is working on, which could bring new combat items (like new shields..)