In regards to work, in regards to becoming a linchpin, you first need to have some sort of expertise, in some sort of niche. I would say that any area is fine, so that people call upon you when it is your area. Well, any area that has value to people that is.
Choose a niche that you like, that would give you a greater chance of continuing, if you want to that is. You should feel free to change things up every once in a while.
Then, from there you can work on finding a niche that is highly valued, and maybe branch out into it every once in a while. If your niche is already highly valued, then you win. If not, make it so you win.