Virtual Aggressor Squadrons?
It's maybe not going to be as good as a real plane as an opponent--what's the modern helmet-mounted display capable of?--but one of the big problems in training is that an opponent in the same sort of plane is going to be misleading. Which is why the USA used the F-5 for their Aggressor Squadrons. Put the "enemy" in a simulator, and they could be flying anything. Even a plane that doesn't exist.
OK, so I was reading a short story by Martin Caidin which had some weird dogfights, and not every pilot is going to be able to switch from a MiG-19 to the Mig-29, and get the most out of the plane, even with the same simulator interface. But if your opponent has different advantages every day I reckon you'll learn better.
(I remember a multi-player flight-sim where everyone was thinking of turning battles, and my only two kills came from fighting in the vertical. And most times I missed.)