Hey guys I know I wouldn't be allowed to publish and get paid for a piece of writing set in the Fallout universe without permission from Bethesda, but what would be classed as off limits?

For example could I publish and receive no revenue? Could I post it online like a blog or something similar?

This seems like a question more geared towards mods. As I don't know the specifics. But I'll answer best I can. You can probably post Fan Fiction anywhere, under the condition nothing is sold. I'd even assume they'd be fine with you having it made into a free book, assuming you have that kinda cash lying about, as neither you or the publisher would get paid. But who here has that cash lying about.. So as long as you don't charge for it, you should be good.

You can post whatever fan fiction you like as long as you don't get payed.