That was my starting point, but I figured most people would want to naturally segregate this stuff by their follow list anyway in the beginning.

Now you can argue that a 10xxx list won't be great either because I may want to have separate lists for different types of content, so maybe use a 30xxx list and give different relays different list ids to use? You could also use a proprietary database that is configured only in that specific relay and doesn't sync anywhere and that wouldn't be bad either. There are many ways of approaching this.