I know I can do a similar function with Playlists, but that doesn’t display in the library.
Feature: I would like to have the ability to display a link to other Library content in a collection. Both the entire series, and/or an individual episode. If a user doesn’t have access to that Library, then the links shouldn’t appear.
Use case: making complete timelines of watchable content that span TV and Movie content.
There’s already quite a number of suggestions discussing what you describe or single aspects of it.
Mostly this should already be possible if you have your content organized in a single show (e.g. with movies as specials). Plex will automatically place the items to your play queue / continue watching hub based on their air date.
I read this before posting, this topic seems to want to span multiple libraries of the same type.
I also read that (it was one of the “your post looks similar” suggestions). I can define what plays next with a playlist pretty simply.
While similar to the two posts you reference, I am trying to display the media as though it’s part of the collection, even though it’s part of a different media library type.
And it does, for a show, or a movie. Not both intertwined.
Example: I have all my ripped Media for a movie, but there are crossover shows that relate directly to that movie story line (like the Marvel Universe, for instance). In that instance, I would want to create a collection with all the movies in it, but place the individual shows, or entire related series in-line chronologically with those movies.
I dunno. Maybe it’s the same. Seems to not be a popular idea anywhow. 
The way I read the 1st linked suggestion, it’s about an option to get cross-library collections. Not primarily to have cross-library collections for movies only (although collections are limited to the same media type at the moment in addition to media of the same library… except you consider the handling of same-name collections).
As you pointed out, the common approach towards what you describe is having a playlist (see Watch crossover episodes of several series in-order). Just trying to wrap my head around your suggestion… would you actually want to add all individual episodes of various Marvel shows in between the movies or should this also allow mixing of seasons or entire shows? I’m still having a hard time to see the actual value add (beyond having a sequential playlist) – which shouldn’t diminish your suggestion… it’s just not really clear to me.
Easiest way for me to illustrate is with this image.
See how the movies are broken up by (especially) Agents of S.H.E.I.L.D., but by other random single episiodes as well?
So I can do that with a playlist, but I’d rather reference the entire season in the places where that would make sense, then then individual episodes where that makes sense, but more than a playlist, I would like it to be prettied up like a collection… if not for any other reason than it is a collection.
And I sure do realize that I’m in the minority probably and I’m wasting a vote. 
I guess it’s a rather specific use case 
