I hate to say it, but now that I looked in the context of the Tweets, I have a thought
If Sakurai only told Japanese Audience they might not know who the character, but didn't do the same for Western fans, then I can see the idea people have brought up that this is a Western Focused character
But he also told people in the West perhaps not really into Smash, but perhaps still into Gaming to watch it anyway, Like they would know the character
If the game/Series the Character is from is more niche in Japan, but is something people are gonna know elsewhere, I got a some guesses
-If the Character is from Western Company, there's good amount that could technically fit. Rayman, League of Legends character, Mortal Kombat character (Scorpion), Even Crash and Master Chief as while they did have success in Japan in the Past (Especially Crash), not the same now
-If it's a Character from a Japanesse Company, There's one main guess I have. Another Sonic Character. Sonic has always been way more niche in Japan than in other Places. Besides the Classic Era, The Last time Sonic games were really successful there was SA1 and SA2. I believe 06 and Unleashed might have sold decently there, but starting with Colors Sonic is not Mainstream like that
I'm telling you guys, I might be winning here. Let's get it