When we talk about falsification in modern science, we usually mean falsification through empirical experiment. It must involve data from the real world.

If you include thought experiments based on nothing but arbitrary axioms, a lot of crazy claims about supernatural beings and other metaphysical curiosities would also qualify as falsifiable. I'm not sure whether you'd like to go down that rabbit hole. Science is about understanding the actual physical world that we live in -- not some fantasy world no matter how internally consistent it might be.