There are countless good and also some not so good ideas that are circling around out there on the internet for future updates for this game.
These are a list of mine. Feel free to share your ideas and thoughts below.

First, there have been many mods out there but none have caught my eyes more than two types. Dimensional and biome. I find that adding to already existing biomes in the game is fine, but honestly to repetitive. My favorite things are seeing new places terrain and special items found only there. I have seen much about a sky update and cave update wishes circling around the internet. Here are some ideas for that. First, the sky should be obvious. Skylands. areas similar to the crazy floating mountains that we all know and love, but on purpose. The difference with these island would be to leave them very high above the clouds far from the ground (or possibly just floating above the void). Then adding more flying mobs that may only spawn in those floating islands. (I know this idea is already out there). For the caves I don't have much ideas. reworking what is in the caves are fine. I personally don't care for anymore underground evil mobs. Plenty enough down there. but some new variations in the way caves look within areas or depths could be a nice change. Mabey some different areas underground like layers like of limestone caves vs crystalline or other natural things. Stalagtites and stalagmites could be an idea.
By far though new biomes would be great. Heres a possible list, swamp jungle mix, deadlands, meteor impact zone, volcanic, rocky zones, varied water heights areas/waterfalls, highlands, skylands, craters, new tree types, bamboo/pandas, penguins, ect.
Dimensional wise, and anti nether like the eternal frost mod. (would love this) Dark world, something like the twilight forest, haunted woods, dream dimension, atlantis like location, maze, darkworld, messed up gravity location, atherial, something like the betweenlands. (maby just make some of the locations from these places into new biomes.

These are just some of my personal thoughts on things. (and I have plenty with detail but don't want to go to much into them, want to leave possibilities open.)