My favorite thing to illustrate are cutaways of science fiction tech. There is virtually no need, if ever, for this as a job and only recently was I starting to do a cutaway of one of my friend's exo-suit designs before a poorly prepared for computer crash and getting a contract job. I enjoy the job I have now as I finally got my foot into the door of professional illustration, but if I'm ever able to recover my old dead harddrive or have time to cutaway other sci-fi tech I'll be happy.
I love technical things and seeing how they work or what makes them work, but the fun for me in sci-fi "technical" illustrations is that there is a bit more liberty involved in creating what makes it tick. That is why two of my favorite illustrators are Richard Chasemore and Hans Jenssen (the illustrators for the Star Wars Cross-sections books).
I figure doing work as a technical illustrator for the next many years will help me not only refine my skill as an artist, but also grow my engineering and technical knowledge (which is very small compared to most technical illustrators) to help aid in producing the illustration I enjoy doing most. I might not earn anything from producing sci-fi cutaways on the side, but to me, they are the most enjoyable things for me to create. Someday I hope to have the right to produce cutaways of some of the famous movie icons like the Iron Man suit or the vehicles from Minority Report or The Island.