Here’s a guitarist (I assume from Azerbaijan) who put some quarter tone frets on his trusty Jolana Tornado.

This guitarist put only one extra fret between the second and third regular ones, and is playing a melody more familiar to western ears.

What comes after more and more frets? That’s right – no frets:

https://www.youtube.com/watch?v=ymTnXVKnvPY

Here’s a video of a guitar with movable (Lego) frets.