Pretty straight forward, but I think it would be valuable to have a plex server backend for mounting etc of media from a plex server. This python project does this, but is not nearly as feature rich as rclone:
glensc/plex-fuse: Plex FUSE Filesystem: Mount Plex Media Server as local filesystem (github.com)
Start with finding API documentation etc.
Then you either develop it yourself, find somebody to do this for you or sponsor development.
Otherwise as there is already queue of almost 100 requests for new backends it realistically won't happen...
What would the point be? I'm trying to figure out what the use case would be for this.
For plex, I'd just use a plex client and play it.