Guitar can be made into a high - tech instrument. For example, it could have built - in sound generators that can create otherworldly sounds, like in some stories where the guitar is used to communicate with alien beings.
In a science fiction setting, the guitarist himself could be a special character. Maybe he's a time - traveler who uses his guitar to blend into different eras and cultures. The guitar could also be a key to unlocking ancient secrets or technologies in different planets or time periods. For instance, in a story, the unique vibrations of the guitar strings are the only way to open a portal to a parallel universe.