There's been lots of trends that spread around the car customizing communities like fiberglass STDs — scissor doors on everything, colossal wings, massive rims, absurd camber — but I'd like to humbly suggest one to try out: trafficators.
Trafficators are semaphore turn signals — little arms that pop out to indicate a turn instead of a boring old blinky light. These were popular in Europe in the early part of the last century, and at car shows you can still occasionally see them on a very vintage Beetle or a Morris Minor or something.
They're goofy and charming, and I think it's absolutely time to integrate these into some modern custom car designs. As long as they flash an amber light, I believe these are still legal, so let's not let that stop us. What makes them exciting is the element of surprise. They could be completely hidden into the bodywork of the car unti activated and then, whammo, it's like a little blinking orange party right there on the side of your car. The turn complete, the little arm hides back into the bodywork, ready to delight anew.
Because of the mechanical, kinetic component, there's lots of novel options to make the act of showing and hiding interesting. You could use the traditonal, lever-arm approach, or perhaps a more modern slide-out type of setup, with a long row of LEDs to announce your serious intent to turn, maybe a delightful fan-fold system with amber EL panels, maybe even a pneumatic party-horn effect for just that right level of class.
Regardless, I'd love to see some modern takes on the old trafficator. If anyone's already seen something like this, post a picture! I have a right to know, dammit!