“First: I do retain and reserve all rights to my work. I’m not very squishy about that fact. Just so you know. If you play in my universe, you implicitly accept I have the right to come around, say ‘mine!’ and then stomp off with all your pretty toys. Yeah, I know. I’m a dick. What can I say.
Second: As long as you can deal with that first point, as far as I’m concerned, you may play in my universe(s) as long as the emphasis is on ‘play.’ This means that nothing you do in my universes may:
a) Generate any sort of economic benefit for you, in any form;
b) Generate any sort of economic benefit for any third party;
c) Cause me economic detriment of any sort.”
Not that I ever expect anyone to ever write fanfic about anything I create. Nor have I ever written any fanfic of any kind (unless you count a DOCTOR WHO story I wrote back in the 7th grade as an assignment).