Now that I think about it, selling what's basically fanfiction has actually been a thing in Japan for years - the so-called doujinshi. Traditionallly sold through the Comiket festival (though many are also available digitally year-round now), they're pretty much fancomics, and their sale is not only completely legal, but many of today's big names in the manga industry started that way! (Example: CLAMP, the group of four authors behind Cardcaptor Sakura and many other series, were originally a bigger group specializing in doujinshi back in the 80s) so most manga authors themselves tend to be very tolerant of the thing as that's how they they began too.
(Word of warning: there are a lot of R18 doujinshi, in a lot of cases being the majority for a given work. Proceed with caution if you want to look them up, especially since by the way Japanese laws work you can literally up anything in them - including underage characters engaging in... you get it. To Japanese law, it's "hey, they're splotches of ink on a page, not actual people, no one was harmed".)
In Japan, the general attitude behind doujinshi is "they're their own thing and each comic is made in fairly limited numbers so they don't take away sales of the original and in fact someone might discover the original through them so it's free publicity!"
Now, even then there are some Japanese right holders which is better to stay away from - Nintendo is one which is very known for sending out cease-and-desist letters to anyone mucking with their IPs. But in general Japanese IPs tend to be a lot more lax on the whole derivative works debacle.