A little late, but I don't think Kiryu has very much fan demand anywhere, which is why I don't think he has the edge on Arle. Comparing their rankings on polls (Kiryu rarely, if ever ranks), comparing their Smashboards thread lengths, and even looking at stray Twitter requests, he's lagging behind pretty bad. Yakuza IS getting promoted pretty heavily, though, and while I've never met a person outside the Smash fandom who'd ask for Arle, I've also never seen the same of Kiryu. So I dunno how much it is "Arle beats Kiryu for fan demand, even in the west" and how much is "fan demand in the Smash fandom only".
You might have Joker as an argument, but Persona 5 curb stomped the **** out of series records and became Atlus' personal piggy bank. Joker also, I believe, had ranked and beaten them in want a bunch. Just not in chance. We also don't know if Sakurai cares much for either of these, or what their directors would think.
Shadow has both of them on-speed dial, but that's mostly on an Echo Fighter (we also have no idea if Assists will be promoted). Tails is a DLC Mii Costume, I doubt they'd sell those twice. Knuckles is both. That leaves Dr. Eggman as the leading Sonic character to fight against them. He's not a boss, an Assist, or a Mii Costume. Unless we get another SEGA character (assuming Akira and Jacky come back with Terry), the ship on SEGA Mii Costumes has sailed. Him being a Spirit only matters for the immediate next character spot.
Between these three (Arle, Kiryu, Eggman) who sounds like the likeliest?