The one thing that I have always wanted in every shop I have worked in or ran was, MORE SPACE.
If you have the space, and tall ceilings get some industrial racking and cover a wall with it, it will change your life. Get a forklift to access the top stuff. You will also be amazed at how oven the forktruck comes in handy.
A gantry crane
Others have said it but get an iron worker.
A computer with a water proof keyboad loaded with FSM's, AllData, and part lookup programs.
A CNC plasma table.
A shower
a shop cat
A really old retired Fab guy who will stand around and tell you that you are doing every thing wrong.