Oh, lol, well I mean I'm American originally as well. I speak the language and I've lived here for several years now but I don't see myself as qualified to make a definitive statement on most of that stuff.
Judging by some of the prediction articles I've read, the really popular picks in Japan that I see coming up regularly are Bandana Dee (as most of you know), Rex/Pyra, Sora, and Incineroar. They also seem to have a thing for Crash Bandicoot because I see people bringing him up in several lists. Shadow and other Sonic characters seem like common requests. Lots of love for ARMS characters too, with pretty much every list saying Springman or someone from that franchise is likely. Check out these (all in Japanese but I've chosen ones with pictures)
https://moving-and-grooving.com/646/post-646/ (Scroll to the bottom for their predictions)
https://matome.naver.jp/odai/2152059317656546501?&page=1 (This one has a ton of picks across 4 pages, lots of them from Fire Emblem and such)
https://www.inside-games.jp/article/2018/07/15/116161.html
You'll notice that none of them even mention Isaac. Unfortunately, while the series is definitely remembered here, I think it gets lost in the shuffle of JRPGs, whereas for a lot of people in the West Golden Sun was one of our early forays into the genre and our first real JRPG on a handheld. Interestingly, at least 2 of them feature K. Rool. One of them gives examples of popular western picks, with number 6 being 6.ロビン(黄金の太陽) (Isaac from Golden Sun). So they do know that he's a popular request over here, and if they know then Sakurai certainly does.
Sorry if that's a bit inconclusive. I've moved fairly recently and don't have many Japanese gamer friends so I can't really give you what the word on the street is, but I hope that at least gives you guys a picture of what the Japanese intarwebz are saying.