Im talking about sports like air gear competitions or like “Keijo”, only thing that matters is that the author has made a sports with rules, be it something completely impossible to play like magic sports or just something that isn’t played in real life but could perfectly be done.
It doesnt have to be the main focus of the story. It can be like what Quidditch is to Harry Potter, just one of many parts. Also, it doesn’t matter how much it takes from real life as long as it is clearly a different thing. (it could be a variation of an existing sport as long as the ruleset is not real)
Having said all that, what are some fictional sports in manga you like?
The only isekai x sport I can think of is the baseball one. Which is pretty good if u haven’t read that.
I think made-up sports are too hard to do. Even Keijo is really just wrestling with special rules. Made-up board games, on the other hand, seems far more common. They had one in Hunter x Hunter (ant arc), Love is War had a few (thanks to Fujiwara and board game club), and Sket-dance had them. Many gambling mangas also have their own board or card games.