Realistically, anything needed to build, maintain, repair, or improve structures is going to be needed. That covers a ton of things - everything from structural engineers to carpenters to painters, etc. Roads, bridges, office buildings, apartment towers, single family homes, cars, boats, etc. And in general, the demographics of "the trades" is older. The US shot itself in the foot with the "everyone has to go to college" thing. And, even if he doesn't like plumbing that much, there's a ton of value in knowing how to do most anything.
The single most competent person I know IS a plumber, electrician, carpenter/framer, roofer, and mechanic (plus army veteran, has done general contracting, historical reenactment, etc). He can pick and choose what he does simply because he can do so much, and there's so much demand.
And he's FI, though he doesn't quite understand the concept of relaxing...
