These are usually called canned messages.
In ages long past (it really feels that way) we talked about a feature similar to this. It allowed someone to create canned messages with optional placeholder fields. Then during selection of such a canned message the editor would force the user to walk over all placeholders for completion.
If you'd be interested in widening the scope of this Proposal I would like to pledge a fair amount on behalf of the foundation. Although I do think a canned messages extension by itself is valuable as well.