Maybe I'm crazy but "gatekeeping" certain characters such as Master Chief, Kratos or any Xbox/PlayStation character only because they didn't appeared on Nintendo platforms seems absurd to me. Of course, having your games on the Switch always will help, but I don't imagine Nintendo and Sakurai being approached by Sony and Microsoft, both offering their biggest characters for Smash, only for Sakurai to be like "wow, would be so big and exciting to have Kratos and Chief... oh, yeah... the Switch doesn't have any God of War or Halo games. What a shame, I can't put them in the game"
In other words, these characters would definitely be the exception to the "must be on Nintendo consoles in some form", if that's even a rule for Nintendo. Sometimes I think some folks aren't really aware that having a Xbox or Playstation character in Smash would be ****ing crazy. Smash is the biggest gaming crossover, put any Microsoft/Sony character in there and you're making history.
Maybe Sakurai would prove me wrong but I'm really that sure that if he had the chance to put Chief or Kratos in the game he would do it, specially if he wants to keep expanding Ultimate as the definitive crossover, and there's no better way to do it than having a guest character from Xbox or PlayStation.