Here, I'll play.
Let's talk about tinting windows. In California, you can tint a car's back windows and rear window with whatever you want. Front passenger windows have to have at least 70% transparency. No approval necessary. What does it take to get an STC to put window tint on an aircraft?
Let's talk about engine systems. In California, you can replace an ignition system with another ignition with any system designated as valid by the manufacturer of that ignition system. Go look at the ElectroAir electronic ignition STC: you still need a magneto system, and that STC was probably quite expensive to get.
Let's talk about upholstery. In California, there appears to be no regulation on upholstery for car interiors. The FAA requires certain fire resistance, and the lab testing is apparently around $1000.
Let's talk about carriage for hire. In California, Uber and Lyft appear to operate without regulation. The FAA would nail you requiring an Air Carrier certificate (Part 135) if you did what they did in aviation.