Disclaimer: I'm speaking as someone who joined recently. Take my perspective with a grain of salt.

I've spent the past (almost) two days perusing the Free, Casual, Advanced, 1x1, and Arena sections. From what I've been able to see, there was a rough balance between original and fandom concepts. For the most part (though I will agree that fandoms did seem a little more common). I also did notice that, in spite of this, quite a few original plots have borrowed similar cliches (ex: plenty of school settings, plenty of gimmicks involving supernatural creatures, plenty of city settings, etc). I don't necessary find that to be bad though as everyone has their own tastes and some genres are just destined to be more popular than others. There were some really, [u]really[/u] cool concepts that I also came across while on here. One of them was Blighted Kingdom which I personally have yet to join due to intimidation factors. :hmm

I'd like to propose that original ideas aren't as uncommon as they seem, but plenty of them tend to borrow tropes and cliches that shove them under the radar. They appear less attractive because they have ideas used time immemorial or come with massive lore dumps involving the world. I feel most players want to feel like the belong to the world instead of acting as a guest, so it's a major turnoff to throw yourself into a world that's already been pre-established. Especially one that you had no impact in making. 

I think they might be seen as less common because they just seem to be less popular, really.