Funny... I use "fanonical" to mean things that fanfic usually assumes happened/happens or that often happen in fanfic, rather than relationships that have no canon basis. Which lead you into the murky waters of what exactly constitutes canon basis, from a canon couple to canon UST to fan-perceived semi- or unintentional canon UST...

But, as for relationships that have no canon basis whatsoever, I have often found myself writing them simply because I like femslash and the Bechdel law is there for a reason. Sometimes it works; sometimes it frankly doesn't... It's all about how those stories and personalities fit together. I guess you could say I prefer writing pairings that do interact in canon at least a little bit, so you have something to go on.