A FUSE loopback filesystem with COW semantics for hard links that could be used with a tweaked SVN client to remove the 2x disk space penalty you pay for keeping a pristine copy of everything and being able to do local diff?
Or a digibox which detected the “ad break beginning”/”ad break ending” signals that British channels send out for VCRs to use, and blanked the screen during the ads?
Or a law which said that patents were non-transferable? So that if a particular company or person invented something, they would be the only ones ever who could licence it, or sue for infringement? This would make patents more likely to be used to protect innovation and far less like a tradeable asset which let you sue your competitors.