It would be useful to have a duration field on bookings like there is on Tasks, that auto-updates along with start and end date.
It would also be useful if there was more granularity to the vacation field, so that bookings could be used to track other forms of time off like sick days, etc. I realize I can add a list field to track this, but I can't edit it from the Crew Planning view.
Also, as far as I can tell, while you can view and edit booking objects using the normal shotgun interface, you cannot create them. I'm not sure if that is a limitation of the app system, or something unique to Booking objects, but it would be useful to be able to create them from the shotgun interface as well.