procedural eyes
I guess this would have to be more of a plug-in/add-on for Daz. Procedural eyes are common, but they're mostly made in places like Maya & blender, would be nice for Daz
http://www.creativecrash.com/maya/downloads/scene-files/c/neyemaker
you can look at that man's video, showing what he made for Maya
It would be nice to have something like that for Daz
The advantage of a procedural eyes? Won't lack in resolution, and will be clean (I'm not sure about most, but when people take eyes from photos, they look ugly and then offer them up as products)
http://www.3dluvr.com/rogueldr/tutorials/eye/eyes.html
a tutorial explaining the procedural eyes & the main shaders at work for it
things that would be good for it, things the costumer should be able to control
1)pupil
while normally black, be nice to be able to change the color of it
and be able to change how much it faded
be able to make it distort in shape (ways to make it something other than round in shape)
2)sclera
change the color
control the veins in the that part (how few, to how many)
3)iris
well, being able to change the sort of details on the iris
the halo rings, speaks in the eyes, that sort of stuff (be able to make them as realistic to cartoony)
I'm sure much more can be done to customize an eyes, but I think that Maya script shows a good way of being able to do that