All too often, even with fantasy settings, that they are based on real cultures with no mixing of other cultures. For instance, the world of LOTR is more or less based on medieval European culture. You're not going to find a ninja in LOTR because even though ninjas did exist in the real world during medieval times they were of a different culture and of a different geographical location. And since LOTR is based on Eurpoean culture during medieval times, for that reason you will not find ninja in LOTR.
Anyway, here is the thing about fantasy, you can do whatever you want, you can have whatever culture or mixture of cultures you want. So you can make a fantasy setting that takes place during medieval times that has knights, ninjas, wizards, anything you want. With Dungeons & Dragons it started out with more or less the same kind of setting as LOTR, a medieval European setting. Then they also came out with the Oriental adventures which was based on the same time period but in China and the far east. The thing is, they were two different settings which did not cross over. With some of the later versions of D&D you could say that maybe the Monk class is a bit of a crossover where some asian elements are introduced into a setting based on medieval Europe but I would like a complete crossover.
Anybody familiar with the world of Lone Wolf by the late, great Joe Dever, his world did have a bit of a crossover. The protagonist whose name was Lone Wolf was what they call a "Kai Lord," they're a bit like Jedi Knights. The story took place in the pretend land of Sommerlund which was, I believe according to Joe, based on a medieval England. However, there were some eastern elements as Lone Wolf knew martial arts and would sometimes engage in exercises that involved eastern meditation.
