I know it's not in the cards, but I'd love to see them update the Speedway at some point and totally revamp the technology. Instead of the reverse slot bumpers, I'd love to see some kind of automated lane keeping system - visual or with embedded sensors. And if they switch to electric motors, they could increase the performance of the cars, allowing them to really get up to speed on the straightaways - especially if they implement variable speed governance to limit the speed entering corners, etc.
I'd guess this is too much to hope for because it would essentially be a whole new attraction and really a whole new ride system.
I'd also love to see Speed Racer IP as I grew up with it, but I could also see it working well as a real user-controlled version of Radiator Springs Racers.
Also, I don't want to re-raise a polarizing argument, but I'm pretty sure the small engines in the current Speedway vehicles are much more polluting than current automobiles - even the so-called polluting hybrid vehicles. BTW, I suspect the reason non-plug-in hybrids have relatively high total lifecycle pollution costs are that they still derive all their power from their internal combustion engine. I also suspect that a lot of folks using EVs also have rooftop solar or are purchasing renewable power, as Disney will increasingly be doing.