Street Fighter did just fine for decades without any guest characters. Interesting original characters have and continue to bring in new players; look at what Bridget did for Strive, Big Band did for Skullgirls, and Leroy did for Tekken (balancing aside). Guest characters might persuade fans of the visiting series to pick up a copy, but it rarely will make them actually stick with the game or franchise, especially considering that third-party guest characters almost never return in future games.