I started thinking about this project after buying a 6-wheeled rover robot. The original intent was to assemble and program it to pick weeds in greenhouse plantations. But after seeing the litter around the town I lived in I got the idea that it could pick litter.
The robot would require advanced image analysis. There should be plenty of training data picked up by for example litterati.org with their awesome app.
A very important aspect of the design would be to keep each unit artistically unique so that the droids would not just be another convenience to throw even more litter at, but instead a positive way to inform about what happens with most the litter (it goes to the sea) and what a difference a little habit-change can do (putting the litter in the nearest trash bin, or better, recycle or even buy produce without wrapping).
I only got as far as beginning to program the movement of the arm before I moved to Sweden, leaving the physical unit behind.