https://en.wikipedia.org/wiki/Perry_Rhodan
The largest entity in SF, its bigger than Star Trek, Star Wars and whatever there is more, it has sold more than two billion copies, the main series is 2900+ novellas, 850 novels of the spin off series called Atlan, 400 paperbacks and 200 hardcovers.
I'm not even going to try and explain the whole thing but the main character is a man called Perry Rhodan and he is the first man who lands on the moon.. and then all hell breaks lose and his adventures brings him to all corners of several universes and spans thousands of years.
https://en.wikipedia.org/wiki/Ayreon
The second universe I like is called Ayreon and its in music, since 1995 Arjen Anthony Lucasses has been working on massive prog/metal/folk/electronic operas kinda which tell the tale of the Forever, the first human like beings, his latest album is called "The Source" and it is about how the human like Alphans become the Forever.
On his albums he typically uses one vocalist for a character, some of his albums have 17 different singers with people like Floor Jansen, James LaBrie, Simone Simons, Tobias Sammet, Tommy Karevik and about every other legend from the metal/prog world.
https://www.youtube.com/channel/UCihO6uVisilu1mn4x2u-HOA
Arjen's youtube channel, he has put the new album online entirely.