btw, I looked into doing this a while ago.
1) a->d converters aren't nearly as easy to do as keyboard encoders... but that should be obvious.
2) variable a->d converters are harder then a static one.
I don't see the need in creating a 100k -> USB because you can already buy them for 10 bucks at radio shack. Probably not worth the time.
I would LOVE to see a cheap 5k -> USB myself. I would buy a few.
Last....
I would REALLY REALLY love to see a true variable one (starting value, and range BOTH variable). Also, make it at least 3 axis (for throddle)... But that would be because it is the only piece missing from my true always tracking lightgun solution.... a cheap a->d converter....