In order for the RPS aspect of AH2 to be realistic, a character should conform to a role. This as opposed to being a Navy fighter pilot one mission, then a Panzer tank commander the next.
For variety this suggest that a single player should be able to have multiple charaters progressing through careers - say a US Navy Fighter Pilot, an RAF Bomber Pilot, a Russian tank commander (hint, hint), etc.
Make it so that only one can be "activated" at a time.
Missions would require certain roles and if you have a character that fills the role, you can activate it and participate in that mission.