The thing most people overlook is that Marth, Roy and Lucas are really more happy accidents than anything.
Keep in mind, the last time we got a JP only character in Smash was Lucas in Brawl...and it turns out that only happened because Sakurai was unaware that Mother 3 wasn't going to be localized.
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It's also been well documented that Marth and Roy were originally planned to be scrapped from all non-JP versions of Melee
because of their obscurity in the west. Of course they backed out on that at the last second.
So really those characters just got extremely lucky. But I don't think it's a "mistake" that Sakurai is willing to do again. There's a reason characters like Takamaru and Sukapon have been made assist trophies instead of playable.