Projector Tail Lights
Not talking blindingly bright tails, but something like fitting a projector kit in the stock tail lights and keeping the regular bulbs. I figured it would be a pretty subtle mod that would look pretty sharp.
The closest example I can think of are the tail lights that come on the Mazda 3, but the light would have a soft, deep glow. Even keeping the red plastic over the whole set up would look pretty good IMO.