Herculean task? Lots of laughter.
I bet there are many nerds out there who make up their own version of skill trees for fun all the time. And it's not even really difficult since there is next to no innovation present. As you said, they just build it from the classic trees and put some abilities here and there. Add some testing if everyone can get everything they want them to have with their available points and voilá, you are finished.
I can't think of a more easy task then that, some intern can perfectly do that. Blizz just acts as if they are splitting the atom.