The problem here is that ninjas + maturity don't mix. Most of everything, besides the original Ninja Scroll, is going to be aimed at kids - because that is who would still be into ninjas at any given point.
You oughta expand into the dramas and mindbenders; the only reasons that, I personally, still watch anime.
Anything by (the late/great) Satoshi Kon: Perfect Blue, Millenium Actress, Tokyo Godfathers and Paranoia Agent. All golden.
There are also a couple dramas I'm really into that are recent or currently airing;
Ano Hana: About a kid who sees his dead friend's ghost and is pushed into reuniting the old clique, for old time's sake. It's kind of like "Stand by me", and make me cry like a little ***** at the end of each episode (and especially at the end)... and I'm a 28 year old man... and only somewhat ashamed of that fact. It was sooooo good, watch it with a chick and you are guaranteed brownie points.
Natsuyuki Rendezvous: Another drama about a ghost seen a by a dude, but in this round it's trying to keep him from hooking up with his widow. Very good show, and great art style.
Kokoro Connect: It's about a group of high school students that are screwed with by some kind of deity into swapping bodies at inopportune times, and later in the season, other things that don't bode well in public. It's pretty good as well.
Desert Punk: Kind of a post-apoc about a dude that sells himself as a mercenary, and it's got quite a perverted streak to it (but it's hilarious). The selling point, for me, is that the main character is the ultimate anti-hero. Only out for himself, and if he gets boned over a job, he'll make everyone suffer for it... and you just CAN'T kill him, he's too slick. One of my favs... and the dubbed version rules way more.
The Tatami Galaxy: A mind-bender that surrounds a dude in college who is trying to take advantage of the best years of his life, and every episode seems to take place in it's own universe with events not affecting the others (so much). It will all make sense at the end of the 11 episode run, so I don't wanna ruin the surprise for anyone. So worth it, imo.
+points to the suggestion of Katanagatari. Art style like it's for kids, but it's pretty brutal and overall a good show. Baccano! and Durarara are two of my favs as well, but it's more for the types that like the disjointed plots of say, Pulp Fiction.
