I would want bricks to be more capable of interacting with their surrounds, like bricks that float in water, being able to get bricks to move when pushed, being able to stand on bricks when they float in water to make boats, making it so you can make levers and stuff using physics.
I would also like to see a baseplate where when you build on it, then hit it with a special tool, everything on the baseplate comes off the brick grid and it free to move around in water, be pushed, and be blown up.
I would also like to see a way to limit the player's turn speed, it is pretty stupid how the rowboat can turn super fast. Locking the player from turning would also be pretty cool for making WW1 tanks with fixed guns only moving up and down.
A boat and helicopter vehicle class would be neat.
A new thing for the water datablock, you can toggle whether or not when you drive a vehicle into it, the vehicle blows up or not. I hate how people would always drive their jeeps into the water and they can just drive all the way to the other island without being spotted by anoyone.
A fluent method of day/night to be coded into the game.
When trying to make caves in terrain, you cannot build in them because it classifies as being burried, which means maps with cool stuff hidden underground won't really work because you can't build in it.
A more dynamic interior lighting system. My space map has a cave inside a planet, and it is pitch black, but the player and the bricks are still really bright.
Better player scaling. Small playertypes can see through bricks when you run into a wall as well as glitch through collision.
Better collision for vehicles. When you fly "helicopters" into a wall, they spaz out and can crash blockland.