In order to be a filler project a project must meet these requirements:
1. Must have not taken that much time to make, less than an hour or so
2. You didn't really make the project a thing and just randomly made it
3. The term "filler project" in scratch means "a project that attempts to fill the gaping hole in your soul that you are desperately trying to fill up because you have no friends. In other words, it must just be a random project.
You can add any projects that meet these requirements. Go nuts...
I know, most of my projects are filler projects, so yeah.