dragons, not anthro dragons, just awesome cool dragons.

tribal designs , of animals are popular.

While this does kind of fall onto the 'furry' side of tattoos, I'd still think about wings and such. Whether they're accents, large tattoos, or even just little small ones, there's always been a kind of... I don't know, fascination with them, if I had to put it into words. Anyways, besides that, I'd say maybe skulls, 'fence' tattoos, like barbed wire and such. Then there's tribal designs, whatever you think of there would probably be good. Other than that, I'd honestly just say start sketching something simple, and see where it takes you. Some people get the weirdest stuff inked onto their bodies, and who knows? Maybe someone will point at one of yours and say, 'that looks sweet! I want that one.'

TL;DR- Draw whatever you want. Nothing is guaranteed to be popular.

Tattoos are usually impressionistic and are either color inked or solid. I think that dragons, Asian and Western are always good, forms to sell to parlors. I do also think you could sell cheap tattoo designs to the individuals here as well similar to what WildSpiritWolf of DA does. As he does all types, catered to the individual, for individual personal use, unless otherwise posted.