yep, written with a k, it would get the [fikzion] pronounciation, emphasis on the second syllable. But I haven't met anyone who hasn't called it fanfiction or fanfic yet, if they weren't talking specifically about fanfiktion.de, one of the two really big German fanfiction archives. Mostly because Fiktion isn't really a word anyone actually uses. It's more academic. If you talk about fiction, it would be 'Geschichten', I think, 'stories', but I never met anyone who said Fangeschichten, either.