Well, in a perfect world, which it'll never be, a Lunar Lander.
In this imperfect world, I've actually been pretty lucky and have no major omissions.
Somewhere in between, there a few I'd rather like to have a go at, most obvious being...
Clipped wing Spitfire - to see how good a piston engined fighter can be
Tornado - to prove they actually can become serviceable, and because I'm fascinating by the aircraft's twin-crew concept
Mignet HM14 Pou de Ciel - just because it fascinates me
SR71 - to see how such a narrowly ultra-high performance aircraft is operated.
Actually, my own aeroplane on a regular basis, without the irritating need to keep earning a living constantly getting in the way - that'd be great!
G