Personally, if I was on the other end, I would be VERY careful what kind of Fan-something I would allow to happen. There is an awful lot of damage fan-things can do to an IP (mostly related to rule #34, but could also be more a racist / violence thing), and a lot of time it can be kinda hard for people to distinguish between official and fan-made content. Thich then paints the original creator in a bad light for something he has not produced himself.
A blanket NO is the easiest way to deal with these issues.
This is basically why brands are very protective, especially for active products. The protections they are granted are strongly rooted in the law, and in many nations (including the U.S.) the protections are incorporated in the constitution and other founding documents.
In addition to fans potentially ruining your brand through sex or immaturity, there are others:
If you are making a series and you are aiming for some direction, it is easy for your fan base to demand a different direction through fan content. Fans are small and lightweight, they can generally make changes far more rapidly than major official content pipelines. You may be steering to the right but your fans build a runaway content that goes viral amongst them and they want to make a strong left. Then you get criticized for not listening to your fan base.
It is both possible and likely for a large fan base to guess the directions you have planned or the likely story lines. Given enough fans some percentage are going to guess correctly. Imagine when (not if) a piece of fan-based content that happens to go viral, but coincidentally your content is very similar to your official direction and was launched immediately before your ad campaign was to start. Now you as the content creator get a public black eye, possibly seen as stealing the fan-fic content or riding their coat tails, and quite likely your product suffers for it.
Very often the big brands will allow and quietly encourage the enthusiastic positive fans on the small scale, as they grow to prominence remind them that their work is unauthorized and they should stop, and if they get big reach for the legal tools to shrink them back down. Posting your own fan-made pokemon card of yourself online is one end of the spectrum, building a pokemon clone is the other. A little bit of positive fan sharing is good since it extends the product line through word-of-mouth. Too much dilutes or contaminates your product.
I think anything trying to relax copyright rules has a snowballs chance in hell of succeeding, unfortunately. 
