As an author, I say, sure you can play in my world. I’m even flattered you would want to. But keep your hands off my main characters!
My chacters are my friends. They live in my head and I live in theirs. They are extensions of me. You cannot possibly know their secrets any more than you can know mine. What they do, what they think, how they act, its far too intimate for me to share with someone else.
Would I steal someone else’s characters? No, that would be like stealing someone else’s friends.
In fact, I would do the opposite. If a fan came up with a character similar to one I was working on, I would curse my bad luck and feel I had to abandon that character. Why, because that character is no longer uniquely mine. Somehow, they have started to dance to the tune of someone else’s mind. They are traitors.
How about a minor character? I might give someone else permission to develop a minor character, if I had no plans for them. They would have to be dead to me first though, because you never know when a minor character might start to develop a mind of their own.
If, though, the minor character were part of a successful series, I’d be inclined to think about licencing their use to you. I will allow you to develop them, give them new life, if you give me 10% – 20% of your profits.
Would I coauthor? Only if I have complete control over the story development, product quality, and distribution. Then, of course, I would want 50% – 60% because that is a lot more work on my part. To be fair, if you came to me wanting to coauthor, I’d be far more interested in helping you develop your own world and your own story. I would rather mentor you to your own success than share, but that is for my own selfish reasons.
Would I use someone else’s minor character? No way. Minor characters are far too easy to re-write, so I would just tweak away until they were unrecognisable. Simples.
If you can develop an existing story in the same way as the original author, you have clearly developed the skills to create your own original works. You are ready to branch out on your own.
Seriously. Re-skin your own characters (authors recycle characters all the time), give them their own story world, and cut out all the original author’s characters. Your idea of the author’s world is going to diverge at some point anyway, so just take those changes to the extreme. Then rename everything and re-brand.
Okay, so your first independent story might still be close enough to the original for the influences to be clearly visible. But I bet your third and fourth ones will be much more original.
Go for it. If you want help, contact me.