WOW! It could be ANYTHING!
If not working on a new design (seldom!) there are always improvements to be made to current designs, and answers to be given to customers' questions. Analyze a failure mode. Improve an interface. Do a cost trade study on a proposed improvement. Ensure the simulations reflect the reality of the airplane...
You will be working in a specific shop (aerodynamics, structures, propulsion, interiors, flight controls, fuel, electrics, air systems...) at any given time. You may be re-assigned to a shop seemingly unrelated to your current specialty, depending on market forces.