@Got a Screw Loose I really like the idea of the extension ports for PWM hubs, but it might be more useful to have an adapter cord instead that can also power 393 motors for competition use. (Maybe sacrifice 1 V5 for 2 393s to help teams with broken motors or shipping delays)
How about more lightweight slide tracks and better linear slide trucks to make linear motion easier? Or just a piece that can slide into the rail slots.
Linear slides and rails really do need reworked somehow. I think adding rollers would help a huge amount. It would let them handle some small amount of bending moment without completely locking up. And having them attach to standard C-Channel, and possibly introducing a smaller C-Channel that is 1X1X1 to go with the current 1X2X1 and the new-ish 1X3X1.
Maybe even make the trucks two pieces so that they can be spaced as desired to ride on any size part, even a 1x5x1.