A few things I would love to see is an airport simulation extending to the many services no one yet seems to be covering:
- Various parking garage options / services at different cost level.
- Airline lounges
- Car rental companies and parking facilities
- drop-off lanes (kiss & fly)
- building levels (ie. 1st level arrival, 2nd departure, 3rd offices/lounges)
- travel agencies
- political lobbying re flying times and airport expansion
- terminal management (connecting terminals, spreading them out, routing certain airlines / passengers)
- Priority lanes and different travel classes
- airport hotel
- cargo facilities / airlines
- customs and quarantine
- fire and ambulance services
- events (incl. natural catastrophes and terrorist attacks)
And of course many other great ideas already mentioned here - eg. In-Airport Services?
as cheese said, you are not the first. Which must mean they are good ideas 