A lot of characters from live action series (Star Trek, Buffy the Vampire Slayer, etc...) tend not to get TF art associated with them, I noticed. I kind of get it, though - it's a bit awkward to see fetishy art based on real people (ie. the actors).
For games, I think a lot of RPG fans have played the Baldur's Gate series. Lots of female characters in those games who haven't gotten the TF treatment, despite some obvious scenarios (all mages can shapeshift; Jaheira, a druid, can turn into a wolf or a bear; Imoen is a Bhaalspawn like the PC, which means she could potentially turn into the Slayer). This exists:
http://shadesofnight.deviantart.com/...ation-76527480 but that's the only BG TF art I've ever seen.