Provide warning for mailings that are being edited to prevent Mailing not saved errors.
In our org, we have multiple users who might need to edit the same mailing. Unfortunately, this creates the possibility that someone leaves the mailing open and someone else opens it and begins editing, leading to the "Mailing not saved. Content may be out of date" error (and tears and hair loss due to lost changes). In our case, this is mostly one user putting together a mailing and another user editing it, fixing formatting errors, etc. It's very hard to train users who are used to Google Docs not to leave the tab open, leading to the need for rigid hand off procedures. We've been trying to get people trained for years, but this continues to be an issue that we waste a lot of time on.
Would it be feasible to add some kind of warning that would warn another user who wants to open another mailing that is already open for editing? I'm thinking just setting a flag on the mailing with a 30 minute expiration, which is updated every time the mailing or content is saved. The flag would be unset when the user exits the mailing. Mailings with the flag would have a warning on the draft mailings screen that would say "Someone else may be editing this mailing" or a warning before opening the mailing. This won't be a perfect solution, but I think it would prevent 95% of problems.
We're using Mosaico, but I think this would be implemented in core and apply to both kinds of mailings.