As for collaboration - I have done a lot, 26 books, and found publishers increasingly resistive to them. It's not that the books are bad; editors won't even read them.
Douglas Adams' Hitchhiker series also shows the potential of lighter fantastic fiction. I read the first, and listened to a tape of a later one, and it's fun.
I did not know at first that it would be a series; I discovered after the first novel that I had more to say about it, so I did another. And another, and then the readers demanded yet more.
I never do a full outline, and if I did, I would not feel bound to it, because the view from inside a scene can be different from the view outside it. But neither do I just start writing and see what happens; I am far more disciplined than that.