2 words...Wood Grain.
Well, I think the PT or G-machine movement has been an incredible step forward in vintage cars. It's more of a form follows function revolution than past superficial and short lived trends. Performance and handling of these cars has been exponentially increased by the aftermarket and pioneers that were looking for more than just straightline advancements. The fact that car makers have caught on to this "trend" is promising, in that, they've invested in the tooling and sourcing of tires for larger diameter wheels. I'm predicting that we won't see this style go away quickly.
However, paint is a different story. There is nothing more subjective or trendy than color, generally speaking. Has everything been tried? Who knows, I doubt it. We've seen solids, pastels and teals mixed with wild graphics, primered retro rods, metal flake speckled lowriders with murals, chameleon (flipflop), Barris style, Gene Winfield fades, skulls and flames of every shape and size, scallops, tribal thorns, deep candies and pearls, Cali-2 tones, 3 tones, race-inspired, and woodies.
Maybe we'll stop painting altogether and have our raw sheetmetal brushed and anodized clear, or maybe chromed or bronzed or upholstered. What about a polished copper plated body? The DOT would probably have a field day with those.
I think there are several "safe" colors that never go out of style: red, black, sliver, dark blue never seem to show their age. The thing is most people want to be different or trendy, that's when you date stamp your car in some ways.