That would be extremely useful in rigging when you have characters with multiple controlers object, and colorizing curves and wires would help organize your scene.
For Example, left controls were blue, right controls were red, middle controls were yellow....

Have a tool which would let you draw lines right on your viewport...
This can help in animation when you need to see the lines of action... so you would draw simply sketches in time...

Tool that would make possiple to snap object(s) together, or side by side, or object to vertex of another object, like so...

So you could make a two empty object with cube centered in middle of them...
Practical example..
You would have two objects - shoulders and hips...and in the middle of them would be controller of stomach area which would be positioned in constant relation between shoulders and hips...
Or you can define a plain by 3 points in the space...
Also good for rigging a complex characters....

Implement Vector Graphic engine into BGE for creating user interfaces in games. It could be connected with game logic editor...
You would have tools for creating vector shapes, or simply import them from a various vector painting softwares....
Fonts, Shapes, alpha, gradients, etc.....

Imagine. Have a tool in blender to decrease your hardware specifications to simulate playing games on the low- and middle-end computers. So you would try out how smoothly players with slower computers will run your game.

Have a node in compositor which would take your picture and give you black/white "mask" of color you would pick from it...
After that you could tweak the RGB curves node and the factor value would be this "mask"....
This could help for example if you render your image and want to turn color of this blue ball to the red... So you don't have to rerender the whole picture

So you could select object, and choose in the tool menu...
Hey I want to control "translate" of this object, or-
I want to control "rotation" of this object, and imediately appers a controler object and his visual representation would be up to you (circle, square, cube, cross,..)
Or lets say , I want to control rotation of this object by rotation of another object...

That would definitely save a lot of time in rigging characters...