I think Aerospace or Mechanical Engineering are great choices as a backup degree for pilots. Either degree will give you great understanding of airplane systems, which will improve your odds at any airline interview in the future.
If your first love (aviation) falls through there are plenty of jobs in space programs or airplane design. Your hands-on knowledge with airplanes and engines will count as some experience in the engineering field. Take a look at the projects NASA, Lockheed, Boeing, and smaller technical companies are working on. Being part of a design team for any of those projects could partially make-up for not being a pilot. I have also seen "Test Pilot Engineer" positions at some of these companies.