I have often doing a collaboration with another author or several fantasy authors.
The trick I think is to find other fantasy authors with similar styles and figuring out how to split the profits from said venture...
With traditional publishing the matter of splitting profits is handled by the publisher and all authors are treated equally.
With Amazon Publishing and similar methods of self-publishing there is a problem however. There currently is no way to split sales between multiple authors, and likewise they have to split up the responsibilities of marketing/etc.
However I think I have come up several possible solutions:
SOLUTION #1: The Simultaneous Trilogy
Imagine that you have three authors who want to write a trilogy together, within the same shared setting. For simplicity's sake let's say all of the action in the stories takes place in a single city (eg. New York).
Each author could, in theory, write their books simultaneously and release all three books on the same date: February 2nd 2022 or something like that.
The books in question could take place in a specific order, or they could all have their events happening simultaneously. The characters of each book could be separate but their stories interconnected in some manner etc.
Likewise each author would responsible for marketing their own books, but would also be able to mention the other books from the series.
It also doesn't have to be a trilogy. It could be a duology of two books or a plethora of writers all writing in the same world. (Similar to the Thieves World anthology series, but without it being an anthology.) In theory 20 or more authors could all publish their books within the shared world at the same time, setting their own prices, using the same logo, book cover layout template, etc.
As a result each author is responsible for their own work, their own
marketing, and reaps their own profits within the Amazon system.
SOLUTION #2: An Open Collaboration
An Open Collaboration would involve one author (or several authors) getting the ball rolling by writing one book and putting it on Amazon, and opening it up for other authors to add their own works in the world. Any author could, without even needing permission, publish their own work set within the world, reusing the logos, branding, etc created by the first author(s) who has created the world and allowed other writers to publish within the world they created.
Again, each author joining the Open Collaboration would be responsible for writing and marketing their own work, but would benefit from being part of a newly established world.
For example H. P. Lovecraft implored other writers to create new literary works based on his Cthulhu creation (which is also currently in the public domain), because he wanted other writers to add more to the story. However the phrase "Call of Cthulhu" is copyrighted by a roleplaying game company, so that specific phrase cannot be used in either books or marketing, asides from Lovecraft's book "The Call of Cthulhu". Otherwise any books that people publish within the Cthulhu world is effectively part of his Open Collaboration.
Thus if someone wanted to they could join Lovecraft's Open Collaboration, or they could start their own open collaboration based on their own world and hope other writers join them.
SOLUTION #3: Shared Amazon Account
This really only works if the authors are very close (like a married couple, close friends, etc) and they agree to collaborate on one or more books and share the same Amazon account, receive royalties via the same joint bank account, etc.
If the two or more authors don't really trust each other I don't see this working. Not without getting a lawyer or a middle man involved.
SOLUTION #4: Collaborate, but Buy The Publication Rights
So this option means the author is taking a financial risk that the book will sell. They hire a fellow fantasy writer to collaborate with, similar to hiring a ghostwriter, and then pay the author for the publication rights to the work produced during the collaboration. The hired author gets paid for their work, still gets their name on the book cover (unlike a ghostwriter who doesn't even get mentioned), and goes on their merry way knowing they've already been paid for their work.
The author who bought the collaboration is then responsible for the remaining writing, all the marketing, and collects all the profits.
SOLUTION #5: Collaborate with a Dead Author
This is a bit more of an unusual idea, but let's run with this...
Find a book that you as an author like which is now in the Public Domain (eg. "The Great Gatsby" by F. Scott Fitzgerald recently became Public Domain as of January 1st 2021) and then co-write a book with the dead author.
Because the book is in the Public Domain you don't have to pay anyone for the rights or worry about splitting the profits.
What is more there are many other books already in this field of Public Domain Collaborations, so it isn't like you're the first person to do this. Every year on January 1st more novels are getting added to the list of Public Domain books. Any number of authors seeking to co-write a book can simply choose an author who has been dead for a very long time and co-write a book with them*.
* It is generally accepted that you are changing a significant part of the book compared to the original book. The characters, the overall plot, the themes, etc will likely be changed significantly, although certain aspects of the book will stay the same.
So which is best?
Hmm. There is no best. There are pros and cons to each of these options.
Can you co-write a book with Jane Austen? Absolutely. Would you really want to? Speaking for myself, no. Not my type of thing.
Some options require more planning, or possibly the expectation that other authors will follow suit, but there is no guarantee than any of these options will be successful in getting readers.
Execution is another factor. What if your writing style and other writer(s) don't have matching literary styles? It might end up being a wasted effort.
You really would need to find a living author who has a similar style and writes similar things, or you are better off just doing your own thing.
Going the traditional publishing route is also always an option, although that has its own set of pros and cons for two authors who want to collaborate.
