1. Working cameras for recon missions.
2. System for directing artillery fire for arty cooperation missions.
3. Bombsights for aircraft that had them in real life.

Those would make two-seater careers more interesting and popular, I'm sure.

4. French two-seaters.

Almost forgot that one. biggrin

"Upon my word I've had as much excitement on a car as in the air, especially since the R.F.C. have had women drivers."

James McCudden, Five Years in the Royal Flying Corps