Problem: On occasions, an edit is made that appears to be good faith, but I want to check it more thoroughly, either when I have more time or better access to sources. However as diffs are automatically marked as visited when I view them, it is not currently possible to keep it flagged to remind me to review it later.
Proposed solution: When a diff is viewed on a page that is in a watchlist, allow a method of marking the diff as unvisited/unread.