In a talk at the Australian Defence Force Academy last year, I mentioned that one option being looked at for future military aircraft was optionally manned (or piloted) or flow with no one on board as a UAVs.
These are civilian or military aircraft modified to be flow without a pilot. This can be useful where sometimes a crew is needed, to fly the aircraft where UAVs are not permitted or where people are needed to do things a computer can't.
At present these are mostly proposals, not real systems. The German built Diamond DA42 civilian twin engine light aircraft, adapted for surveillance., is offered as an "Optional Piloted Surveillance and Reconnaissance System". Further in the future Lockheed Martin has proposed a pilotless F-35 fighter.
Recently Boeing has proposed an optionally manned Gulfstream G550 business jet for the US Navy’s Broad Area Maritime Surveillance (BAMS) project. On a smaller scale, the current crop of very light jets (VLJ), would seem suitable. These have advanced electronic avionics which are adaptable to remote control, carbon fiber construction which can be modified for sensors and are intended to be produced in large numbers at low cost (starting at $1M). Most have two engines, but units such as the Eclipse ECJ have a single engine.