The ability for the cleaning service to be notified with a daily report of check-ins/check-outs. In fact this would probably just be a report and if reports could be scheduled to send automatically that would be perfect for this. Even better would be a kind of service through maybe twilio.com the cleaning service can text like "CLEAN 2" to say room 2 is now in the clean state.
I imagine a lot of these features could be sold as add-ons to the main package. I would gladly pay an extra $10 per month for this kind of "add-on" service.