I wonder if they'll incorporate some kind of "augmented reality" into the attraction now - kind of like the new hitchhiking ghosts. You design your new car, and then as you pass through the attraction, you go in front of video monitors that overlay your standard-issue TT car with your chosen design. So, you could have the appearance of seeing yourself ride around in some very weird looking designs, maybe even get a ride photo in the augmented car.
For the selection process, I wonder if it'll be a voting scheme like the old Horizons, where you could vote on the ending you wanted to see. Or, perhaps you'll get to select one feature of the car, so each of the six occupants gets a voice in the matter. It'll probably be something like SSE, where you select options as you go, and then the final ride incorporates them.
Let's think about what they cannot change, at least what is not likely to be changed in a short-term refurb/overlay. Almost certainly there will be no track layout changes, nor vehicle changes, except for a SSE-type fix that adds touchscreen monitors. In terms of the ride itself, each car will probably have to stick to similar profiles for acceleration and deceleration, to keep the timing of the vehicles in synch, so the user-driven inputs probably won't affect how the vehicle drives on the track itself. That's too bad, because it would've been nice to make the actual vehicle movements interactive.
Perhaps there will be a portion of the ride where you can select a test environment, and then ride through it? Can Imagineering pull off a convincing projection of zooming through different environments (snow, forest, coastal) using something like what was done with the "If you had Wings" attraction in Tomorrowland?
If nothing else, I'll be glad that the next time I ride TT, it'll probably have all of its special effects in working order!