A couple things that have been on my mind for a bit:
For sound emitters, could we have a way to start from/seek to a specific time in a sound? There's soundTimeline scripts to set specific times you can seek to, but they only work for FMOD events, which afaik mods can't create.
VehiclePart:setModelVisible() doesn't apply the transform of the vehicle to the new model until the vehicle updates on the next tick; this is really obvious and distracting at 60fps. Attached is an image of what this can look like. The only way I've found around this is to call the vehicle's entire update method after setting model visibility but that's probably not a good idea?
Some way to control shader uniforms from Lua would let us get a lot more creative with visuals.