My hypothesis is that a dog would totally exhaust itself if its humans would be so good as to crawl under the furniture and retrieve doggy the ball. Humans, though, have an aversion to excercise and particularly the crawl most noticeable after eating large meals. To serve all the masters equally requires the patience and readiness of, obviously, a machine. That's where the Roomba™ enters this scenario -- always ready and ever patient.

I propose that the Roomba™ be fitted with a roll-under fetcher and a centrally mounted active bumper mechanism sufficient in power to eject a tennis ball. The quick responding robot thereby outfitted would be programmed to park itself just out of reach under the obvious piece of furniture and remain there on standby. Any stray ball actives the bumper and the ball is ejected back up its path, and if designed correctly, straight back into charging dog's range.

I personally think this would be oodles of fun, and think further expressions of this basic design will evolve.