I gotta say, if you draw Nico Robin, I will fight anyone to get that piece.
I like your art style, and I think alot of people will, it looks like you do different styles, from skimming through your site, the fanart and the sketchbook pieces had a variety of styles and that's something really good, you can use different styles for each series/subject and it's sure to get someone's attention.
I know people will say to do the popular and the newest characters but some of us prefer to stick with old favorites, or the rare shows, so I'd suggest:
Nico Robin(I look everywhere for stuff featuring her and rarely am successful)
I'd say Sailor Moon but do it in a different style than anyone else's to make it stand out more(like base it on Da Vinci's women or Botticelli's women, You know?)
I might be forgetting other animes, but those were really really popular a few years back and I know I miss them.
Or do the newest shows, (I'm behind on new shows myself, actually) but I am a rabid fan of 'Princess Jellyfish' and "Kore Wa Desku Zombie?"(or however it's written, heh) so maybe stuff from those will sell well?(I know I'd flip if I saw a Yuki piece, she's so elegantly pretty)
Or do rare shows that have cult status, like Kyo Kara Maoh or whatever. Oh! Or even Japanese movies, like 'Survive Style +5' had a woman with these really cool outfits, someone might like seeing an artist's take on her outfits, or 'Rookies' or or HanaKimi, something. In my opinion, everyone does the same things over and over, so it's a relief to see someone do something vastly different, whether it's in style or in content, so that's why I'm suggesting old animes no one talks about anymore, instead of Naruto/Bleach/DragonBallZ/Etc. I'm sure, though, no matter what you choose, you'll sell well, you have a variety of styles you can employ to appeal to different aesthetics.
I hope that helped?