I think a wide variety of hands would be your best shot at actually making some money. While shinki's come with different hands already, there will always be some hand positions that will be wanted, make them compatible with other line's weapons, like Gunpla or GI Joe or other action figures, and give them alternative positions to the pre-existing ones. However, hands are the most difficult to model, so your next best bet is to come up with a bunch of different shoes or something. or you could make full bodies and sell them as nakeds, those would sell like hotcakes! but again, not easy to model. regardless, you'll have to come up with either full bodies, replacement parts, or alternate parts/armors to actually sell, just making printed limbs is cool, but not likely to sell especially well.
If you could model up the two bike girls armors
that konami designed, prototyped, and never released, that would be awesome! i'd buy that in a heartbeat! but it'd be a huge project.
anyway, cool idea and good luck with the joint issues, shapeways isn't great for tight joints.