I think Terai Yuki would be particularly tough, because she's not only a really old figure, she's no longer available, unless you want to try to buy from a Japanese web site. Perhaps there would be some interest in Japan, where there could be some nostalgia value, but marketing would be an issue if you don't speak Japanese.

I fear the Poser market in general has shrunk so much, it's already a niche. Poser standalone products can typically be used in DS as well, and it would probably be smart to offer DS support if you can.

That said, there would be some older products I spend money on. More textures for K4. The few available are really old, and look it. The mapping isn't quite the same as for V4/M4, so even if you can find adult textures without makeup or beard stubble, they don't look right.

Stuff for DAZ's chibi figure, Nana, which is pretty unique in Poserdom. Maybe some stuff for other toon figures, like Sam and Sadie.

Oh, and if you're doing standalone figures, I would love a cute cat figure, like in Secret Cat Forest. Definitely toony, but not anthropomorphic. A few cat-like poses, and a million different coat/eye colors (all sold separately, of course ;-).