I personally would like a mining update. Possibly biomes underground (which would be awesome and give another level of immersion) and maybe a new ore that could go between iron and diamond as we haven't gotten any new ores since Emeralds in 1.3. I currently think that underground desperately needs an update. I would love something like stalactites and stalagmites which could glow or have some sort of new block.
I also like the idea of glowing mushrooms that grow underground and give off light and could possibly be a new brewing ingredient? I think it would be cool.
I also like the idea of an update centered around the ocean and other bodies of water. I would like to actually be able to see the fish in the water, and you could put your base near parts of a river or shoreline where fish are more densely located. Eventually, you could end up having to adventure farther out to find more populations of fish to catch.
It could be beneficial to add actual ocean structures to make them look more alive. You could add coral reefs, clumps of seaweed, sea anemone, ocean caves, and shipwrecks. Not only would it actually make the ocean not as boring, but it could actually give a purpose for underwater bases.
They could add sea creatures such as sharks, octopi, crabs (new food?), jellyfish, stingrays, eels, sea urchins, and sea stars. I would also like to see dolphins or turtles added as a tameable mob that you could ride in the ocean as another form of transport. There could also be mobs that are only in certain sea levels, and they get creepier the farther down you go. An interesting feature to add would be a Kraken boss, which you have to fight. If you come across it in a boat, it should be able to break the boat into planks and sticks, and drag you down to the depths.
There should also be deep sea vents the farther down you go, which could damage the player if they get to close. Somehow you could be able to collect a new type of fuel that could last longer than lava, but there would be more risk to get it than lava.
Sorry for the long post, but I was bursting with ideas!