In truth the vast majority of fantasy and sci fi worlds would fall apart because they are deeply logically inconsistent on very many levels. However you could ignore that and still set the world to be what it is... and somehow make people ignore or forget about obvious things. Like if healing magic exists why are people ever sick, why has technology not developed way further, if transmutation magic exists what is the value of gold if i can make it out of iron etc. Just pointing this out, i am as excited as anyone to explore these worlds! Makes me wonder what such a logically consistent world would look like and how they would operate.