Didn't realise this thread existed.
Curious.
I will say this - fan fiction is largely cursed from the start by its very name. 'Fan fiction' sounds bloody awful and as most of it is bloody awful then that is what the general public assumes about it. The fact is that it is immensely popular, although you will still find few people admitting to reading it or writing it. It is a very secret, guilty pleasure, admittedly. When I first discovered it, I thought it was bizarre and freakish ... (I still think much of it is) ... but three years later those of you who know me will know that my opinion has changed somewhat

But derivative fiction (as is probably a more appropriate term for it) is actually a fantastically liberating way of exploring imagination and writing skills, all from the anonymous security of your computer. Yes, fanfic writers use characters they are familiar with and whom they know readers will want to read about. It's an easy starting point. Yes, a lot of fanfic is wank fodder

, and who the hell are we to dictate what people should fantasise about in the privacy of their own heads and houses as long as it is safe and harmless and kept private. But, anyway, a hell of a lot of it is not erotic in any shape or form. And the way fan fiction works means that you get immediate feedback - OK, that feedback is largely from a captive (not always discerning) audience who are already on your side because you're writing about characters they love - but if you sort the wheat from the chaff it can be very liberating and edifying. I would encourage young writers to have a go at writing it if they want. Yes, make sure they keep it age-appropriate, obviously, but I think it's a great way of developing writing skills and exploring what styles work for them as writers. And if they post to the right forums they'll get some very constructive feedback. I know very little about Pottermore but it sounds like this is something which JKR has rightly latched onto to enable young people to have a go at their writing within the HP universe.
Going back to the original point - derivative fiction should and must be about FICTIONAL CHARACTERS. Fictional characters are public goods, real people are not. Fictional characters are also relatively limitless in what they can achieve. Real people are not. We may have an image in our head of a character resembling an actor who has portrayed him or her as a starting point, as a muse or inspiration possibly, but after that it is about character, simple as that.
The 'cheat' of fan fiction is that the writer does not have to work on getting their readers to warm to the characters. That has already been done for them. Creating an original character from scratch ... that's the really tricky bit.