This might sound silly, but ever hear about the hurdy-gurdy? I was reading an article about it and just got an interesting idea of how it could be adapted to guitar. The hurdy gurdy is driven by a crank that turns a rosin-ed wheel that rubs against the string(s), causing an effect much like the continuous bowing of a string, and there are keys played that essentially fret strings at certain intervals. Imagine installing a miniature version of that wheel with a small electric motor (whose speed is controlled with a potentiometer) and hooking it up under or over guitar strings- it would be an effect much like a sustainer, ebow, or an actual bow, but different, too.
It might sound like a silly idea, but considering how so many other innovations (shapes, materials, electronics, etc.) have been pursued already to the point where they're considered more as "options" than bona fide "innovations," maybe the way to go is thinking about innovating in the way the guitar is played.