Well, Fire Emblem Heroes did get a bit of content in Smash Ultimate in the form of two music tracks(while the main theme existed before the game, the new remix is very much based on Heroes' version). Mario Kart 8 Deluxe got the Booster pack that had courses from Tour and Fire Emblem Engage also had Fire Emblem Heroes content, including one of the games' main characters as an Emblem. So I don't see mobile games getting ignored by Smash Bros.
Granblue Fantasy is certainly possible. If Saber from the Fate series gets into Smash Bros, definitely expect some content from Fate/Grand Order. Genshin Impact is another one that is absolutely possible. And of course, Fire Emblem Heroes could get more content in Smash Bros. besides music and Mario Kart Tour could get some music or content in Smash Bros too.
Boy oh boy, if some of you thought the reaction to Steve was very divisive, imagine Red from Angry Birds getting in Smash Bros before another Sonic character, let alone another Sega IP like Puyo Puyo, Yakuza, Shin Megami Tensei, etc.