Plus one for this feature. The module would have to be made so that it can be applied to any service.
Custom Fields - Ability for App Admin to add Custom Fields (so that the he/she can add fields that request the information that they need from their respective customers for the booking).
Reminders - A Push Notification is sent to the App Admin and the Customer reminding them of the booking.
App Admin App UI - A booking UI for the App Admin in the app... So that while in the app the App Admin can view their bookings, edit their bookings, delete a booking, etc.
Customer App UI - Ability for the customer to view their bookings.