I want to talk about the idea of Brandon Sanderson's Cosmere.
I find it facinating and It makes me wonder if any other author has done this before outside of a comic book. It's even more facinating when you consider the confidence the guy had to begin introducing elements of this idea ever since his first published book. The fact that hes such a prolific writer, who sells and produces a great story, makes the goals of the idea more than just a pipe dream...unless something happens to the guy and all his secret writing clones, its going to happen.
To people new to the idea, Brandon Sanderson has published several series of novels, each series taking place on its own world and timeline, with its own unique magic system. However he is weaving elements and hints into these series that link each series to an overall event that created threads of power (shards), that came from the destruction of a god. Each of these 16 pieces of the god were seized by the people in the group that killed it, and each became a facet of a god. They created worlds, magic, and people (basically each having a series, or sometimes two or more being on the same world, usually in contention with each other). The elements being woven between the worlds are the occasional mysterous appearance of a character or item that comes from a different series. We are just Starting to see small details of the overarching series that has yet to be written.
Now the closest I have seen this attempted would be a Marvel vs DC crossover. I've never seen it happen in novels.
The Series that have been linked to the Cosmere:
The Stormlight Archive (The way of Kings, Words of Radiance, Oathbringer(pending) (more pending)).
Mistborn Currently at 6 books, 7th and another trilogy after that
Elantris (Only 1 book at the moment, more planned).
Several small novellas