I'm kind of surprised to see so little Kiryu support within the Smash community, maybe beacuse the sereis only recently has mainstream appeal to western audiences. Anyway Kiryu I can totally picture getting in the seconds fighter pass, seems like the "out there" surprise pick in the same vein that Joker was. Yakuza is pretty big and is a continuously successful franchise, up to 18 games now including remakes and spin-offs.
I'm mainly familiar with Yakuza 0, so i thought it would be cool if he had the different fighting styles from that game n smash. It would kind of work like Monado Arts in that you can switch it, but not stuck on a time limit and The different styles give Kiryu slightly different attributes in addition to a few unique moves.
would be his standard style, maybe get a bonus to thowing and using items while in this style.
could give him a little more movement speed and a better jab combo but weakens his grabs while in the style
would make Kiryu a little heavier and give him some armor on some of his attacks.
Could be developed more, but honestly as long as if you can Smash people with a bicycle with his move set then it's perfect. Despite Kiryu being such a serious character, he often used as "The Straight Man" for the wacky humor of Yakuza, seeing his serious self next to Kirby and Pikachu would be hilarious and actually fit the tone of his series.
A traveling stage around Kamurocho would be fun, especially if you had multiple Majima cameos. Music would be amazing to be fight to, there's a plethora to chose from. A more obscure choice for the songlist would be some stuff from Kurohyou: Ryu Ga Gotoku Shinshou (Yakuza: Black Panther), it's a spin-off game but Hideki Naganuma of Jet Set Radio fame made some of the music and would love some more of it.