Sure, I added a pairing tag as well. Historically we don't split these for ease of access. As an imperfect example, the majority of Virgins' Empire art here is fanart, and I wouldn't really expect the average viewer to specifically search outside of the series to find art hosted on the site (even if they would, I'm not sure this is a nice design). But historically the tradeoff wasn't that big, considering few series had much external art and the "art that became a series" thing wasn't a trend yet. So maybe it's time for a change. I'll think about it and stuff. Thanks.
Thanks, that was fast. I believe it is most intuitive if Series tags stay with the official series, whilst third(ish)-party work should be Doujins. Perhaps it's possible to automatically have a button on each series' page that links to related Doujins if they exist?
With series like Saki, Akuma no Riddle, Yuru Yuri, Bocchi the Rock!, etc., where both the original work and derivative manga are hosted here, it'd be cool if there was a simple, unambiguous way to relate them to one another without treating manga and images differently.
