It is a bit rough in my head but as a guideline
Features that I do not see in core, but are fine in a plugin
- Assign user to topic
- Stop watches to measure how long a user worked on a topic
- Custom list of statuses for a topic (open - in-progress - requires help)
- Elaborate reporting dashboards
- Assign priority to topic
- Due date
Features that I see in core
- Way better handling of messages
- Better messages UI, suggested topics for messages
- Presence features (who read the message, who is reading it, who is typing) - opt-in feature needs to be in core to hard to integrate from the outside
- Canned replies
On topic I can see either echo or optional “receipt of topic” a feature we have for improved mail support.