After Caves, I would definitely like to see some wild, expansive, New-Dimension type stuff.
But there's probably some existing content I'd like to see updated first.
Most importantly, terrain generation, which is in a pretty poor state atm. In tandem with that, updating biomes, many of which are really neglected and just not fun to explore. The biome vote events are a start, but I wanna see, like, 10 times that.
The End Dimension is also a little lackluster compared to how developed the Nether will be after 1.16, so that should be updated too