If you could make any add-on you want without a engine change...

[zmaster]

#1 destroy the add-on limit (4000 something currently)
#2 water that actually flows
#3 allow all bricks to move with some kind of "engine" brick
#4 get shaders to work on my graphics card (Before you ask, it's compatable with with any other game that uses equivilent shaders and does support GL_ARRAY or whatever the console was complaining about)
#5 increase mini-nuke size by a little over 9000 (just kidding)

[Ad Bot]

[e-maxx]

remove the brick grid

[Wheatley]

I'd make something that determines what players collide with, such as other players, bricks, etc.

I'd also make projectiles have bounding boxes. nvm i completely derped and didn't see "out" in the title

Yeah I don't know, I pretty much already can...

[Brian Smithers]

Oh god
You can only dream of the stuff Slicksilver would do

[Subpixel]

remove the brick grid

That's kind of changing the engine

[gr8dayseth]

A humongous lego weapons pack with at least +100 weapons.

Ranging from:

Swords
Bows
Crossbows
Other melee
Flintlock weapons
BrickArm weapons

Or a BO2 scorestreak pack.

[lcyGamma]

Cameras and mirrors which display on print bricks.

[Demian]

I would be quite happy with just major optimizations and increased brick limit.

[Barlz]

be able to rotate bricks off the grid.

[Remousamavi]

Buildable vehicles using live bricks with events; builds that can be flown or driven and aren't fixed to the grid.