- Email messages when someone is mentioned but are offline
- Integration with source control, CI, and issue tracking systems being used, targeted by channel/group
- Ability to filter channels by conversation/users (includes some way to attach and track conversation topics on messages in the same channel)
- Web app with desktop notifications and ability to mute those notifications by type
Do the hard work of making these easy to configure and use, and you may be able to compete with hipchat/campfire/etc.