Here is what I suppose to be an easy to implement feature (correct me if I’m wrong) with great potential: multi-group mentions. More precisely: multi-group mentions based on a logical AND conjunction, @groupX∧groupY, as it were. In plain English: the message will be sent to all members who are a member of both group X AND group Y
Unless I missed something, currently you can only send notifications/messages to groups based on a logical OR conjunction, i.e. by simply mentioning multiple groups.
So what might a multi-group-and-mention be useful for? For example, imagine a multi-lingual site, where speakers of a specific language are members of dedicated language groups. Furthermore, they can also me members of groups dedicated to specific interests. Multi-group-and-mentions would make it possible to address, for example all French speaking members with a specific interest.