Suggestion - Shared, Live, Editable Content
A checklist shared between a number of contacts would be very useful. Possibly solved by 'pinning' the share, so it's always the most recent message. Possibly another 'view' of 'shared' or something.
An example is 'gift ideas' or 'vacation plans', 'things to do here in Mexico', voting, polls, etc. Could be markdown. Could be 'pinned'. Could be 'unpinned'.