Definately for more, cooler gears. More sizes of spur gears, worm gear, rack and pinion, bevel, etc
mecanum wheels
More wheel sizes (including wider width wheels)
Removable/replacable/variable wheel treads (for replacing worn ones, adjusting tractions, etc)
3-axis joystick for vex controller
more powerful motors (come on, you know you want em
)
More sprocket sizes
Smaller shaft encoders