I don’t know if the solution is to have a tag attribute but I agree with solving this problem. There is absolutely no way to define a watch order for a set of movies.
Why the current Collections feature doesn’t work:
Collections do now allow you to custom sort like playlists.
Why the current Playlist feature doesn’t work:
Playlists come very close to achieving a watch order. Its flaw is when a movie file is changed (like when its quality is updated). I made a similar post here: Keep media in Playlist when file changes
