I was thinking mostly about air-to-air combat. Other missions (recon, bombing, ground support, refueling, etc.) can or are done with UAVs now. But on thinking about it further, the UAVs of tomorrow can be much faster and more maneuverable than any manned fighter aircraft once you remove the requirement of keeping the pilot alive. I don't know that it could be done today, but I imagine in the not-to-distant future we'll have effective air-to-air UAVs.