No one knows that Fastlane is even an official map, but it could be pretty interesting to see what someone could do with it. I always liked how aggressively close the houses are to mid, and how open the second point area is. Everything past the shutters to last is just way too open and boxy, but I'm sure someone could come up with some very clever additions/removals/fixes for it.
The only other 5cp maps that haven't been touched (afaik) are 5gorge and Foundry, both of which have many large inherent flaws that wouldn't be worth fixing.
I forget if Well has been tested competitively at all, does anyone know about that?
If you want to go beyond 5cp, I've always been interested to see if some of the Arena maps could be made into KotH maps and then tweaked to be competitive. We need more good KotH maps, especially for 6s.