I believe it's a grey area. So what happens entirely depends on the copyright
holder. They can disappear your work any time. On the other hand copyrights
unlike patents haven't got any defend or lose it clause so they could ignore
it too. Often this is what the do until some work gains popularity...
I personally wouldn't put any effort to something like this. It's building on
quick sand.
This article should be useful but I haven't read it
https://en.wikipedia.org/wiki/Legal_issues_with_fan_fiction