I've had a similar conversation multiple times with several different friends - though usually around skills instead of knowledge. The desired skills always seem to boil down to trades like auto repair, masonry, or carpentry. Presumably because I spend so much time in-doors, in front of a computer, creating "things" in the abstract, instead of creating something tangible with my hands.
Macroeconomics, German, Italian, wine making, sailing, playing a guitar, auto repair. I feel like I've scratched the surface on most of these and wish I had time to dig deeper. But I also have this constant feeling that what I supposedly <i>do</i> know well isn't enough and must improve.