There’s an existing thread that’s dealing with a very similar suggestion.
While the OP is focusing on how to show/indicate missing episodes or entire seasons while displaying that particular show, further down in the thread there’s also a discussion how this can be identified on the higher tiers of that show or library.
This appears to be pretty much the same to me – in that case I suggest you vote/comment in that thread in order to avoid distracting votes/attention (and closing this one as a duplicate).
