While the existence of a Western third party seems unlikely, I'd like to offer as a counterpoint:
We as a community are really good at making rules simply because they follow trends that haven't yet been broken. Case in point:
- Smash will only ever have Nintendo characters! We'll never get third parties in! (before Brawl)
- Smash won't do DLC. Nintendo isn't EA! (before Sm4sh)
- We'll only get one third party per company, so Ryu won't get in since we have Megaman (before Sm4sh DLC)
- All of the DLC fighters will come from the ballot, so Geno/Isaac/Banjo are in boyz! (before Sm4sh DLC)
- There's no way they'd add Corrin. They're too new! (before Sm4sh DLC)
- Pichu will never return since there are too many Pokémon! Transformations will never ever return! Toon Link replaced Young Link! (before Ultimate)
- K.Rool is too irrelevant! Ridley is too big! They're not big in Japan! (before Ultimate)
- We'll only get one third party character per franchise! There's no way we're getting Ken! (before Ultimate)
- We'll get an echo for third parties, so Shadow 100% confirmed!
- Box theory? Sakurai totally lied when he said we weren't getting many newcomers! The Grinch it is!
We've made fanrules all the time, and none of them hold consistently. So yes, we haven't gotten any Western third parties. Yet. But we have gotten Western characters (
). Nintendo also knows that the series is big worldwide - he decided against Takamaru in 4/Ult since he's obscure in the west, but that didn't stop Marth/Roy/Lucas in Melee/Brawl. Maybe his worldview has been expanded in those 17 years? We don't know. We do know that collaboration between countries is getting more common, so it's much more likely now that there are decent lines of communication and translation between the two.
Now of course I'm on board for a western character (one in particular). The leaked bunch don't excite me since I'd rather a different Bethesda character, but Steve from Minecraft makes too much sense to ignore. That game's been going strong for 10 years, and even Sakurai likes it. I can't believe that he'd just come out and say "Welp, he's American, so **** him and his legacy."