I like the black low-rider S10. Good look.
As far as getting CRAZY with the back of a pickup truck....
Beds DO come completely off. I've always thought it was cool when somebody did a wood bed, in that classic pickup truck look.
It would be completely possible to make an entire bed/cap combination from scratch. That would potentially MAXIMIZE aero-improvements. It would just be a LOT of work!
An advantage is that since beds just bolt on, you could have a stock bed AND an "Aero-Bed" and (relatively) easily swap the two out.
That's a bit beyond what I'm planning on doing right now, but I'd love to see somebody do a complete from-scratch pickup back end like that.