I personally think that more mobs would be nice. It would be great to have some brand new monsters and animals with new mechanics to make the game more interesting, but I also think there should be more types of existing mobs. People hated Drowned in Update Aquatic, but I think mobs like that make the game more natural, having a zombie drown and turn into a new mob. Maybe some animals that have different looks in different biomes or have some special types.

That brings me to more biomes. I believe some new biomes would be nice. Not that we know that they are capable of better terrain generation, I think there are more possibilties. Maybe some rare biomes with crazy terrain, or some modified versions of existing biomes.

The thing that needs to be added the most is new content for the nether/end. The end hasn't been touched since 1.9. The nether got magma blocks and some new crafting recipies in 1.10 but there's been nothing else added since it's release. That's literally 2 thirds of the game that haven't been updated in years.

Also, the entire underground needs to be updated. Different stone types, different cave formations (open caverns, stalactites/stalagmites), that kind of thing. No more ores or anything though.

What I meant by none of the above as I voted it, is more building blocks like limestone, basalt, and maybe even an actual marble block. I don't think it's feasible, but I want something like the Chisels mod, except more simplified.

Chests of each different wood type would be so cool! I also think Minecraft needs eagles and a volcanic plains biome.

More redstone ._.

pizza.

i want birds!!! :) not just parrots... and also Chest types would be nice! and from weapons maybe crosbow? :D or flint rifle... :)

They should add more ores

Medieval siege weapons!