I'm no lawyer, so take this with a grain of salt (it's mostly my opinion) :

Why should it be a problem if you post a work you did youself? Even if it is of a world not of your making. As long as you do not claim it is your original work and do not sell it, there is no problem. I'm pretty sure that in switzerland (where i live) no one could take action against you.
If you start to sell it or use it in published magazines or similar venues, that's a very different thing.

As a customer, i would vote with my wallet if i found out that a publisher (WotC etc etc) goes after fans who put up their fan-work for free. They make all their money from us fans...and then they try to screw us over?! Yeah, no thanks...i will never buy from you again.