Instead of inventing all of the modules from ground up allow players the ability to play other simulations they have and use the time they spend in there, the money they make to be used in game.
In other words allow careers from other sims outside of yours.
I would love to be a pilot, so... let me use Microsoft's flight simulator x, or Lockheed Martins' P3d or X-plane to fly my routes there. Perhaps you could build a module that would interface with said simulator. No offence but you're not going to make any better flight sims than what's out there now.
Don't stop there, how about truck driving, then tie in to an existing truck sim platform that is currently out their.
Same with trains, use train simulator
ship - ship sim, etc.
If you could pull that off that would be awesome!
It would help these other sims anyways. Once your flight is over in flight simulator, it would help to flesh out the other non-flying part.