SMS/Messaging apps would be what I would primarily be interested in to start, (as far as purchasing) but I could probably think of a bunch of other things. I am considering selling some apps as well, but like Fastdevice said, I would want to know the audience reach. Will the app store apps be open to people on open source installs, or just the 2600Hz hosted/managed/private cloud systems?
So as far as top three for purchasing:
1. SMS/Messaging/Notifications
2. CRM Integration
3. Switchboard/Operator console-style apps.