I think roster wise it's a bit tricky to think of another hook except for maybe the third-party heavy hitters that haven't appeared yet and even Nintendo might be reluctant to get into the practice of escalating the sheer scope of Smash in negotiations for that. I actually think it might be more prudent to further expand on the companies they're working with now, especially given that Nintendo's in a stronger position now than they were during Ultimate's development.
Given the Japanese market has becoming increasingly dominated by the Switch and that will likely continue with the Switch 2, it's within the best interest of Nintendo and the third parties they're working with now to keep that synergy going. Konami, SE, Sega, Namco, Capcom, all have enough IP's to appeal to Eastern and Western tastes, and there's numerous scenarios where a Simon/Richter or Hero/Banjo style double reveal could effectively maximize the hype in the two most critical regions for Smash sales.
Western companies certainly have some great options, and the likes of the Doom Slayer, Rayman, Master Chief, Crash, John Marston, etc would all do solid numbers. However, if the easier/less expensive option is to continue with the companies they're working with back home (Microsoft with Steve & maybe Banjo excepted), that's a consideration that Nintendo might very well keep in mind for Smash.