Smash Bros Utimate wants to be the crossover ever created.
Monolith Soft already has created a major crossover under the Nintendo leadership. Project X Zone 2 already has brought together characters from Ace Attorney, Devil May Cry, Fire Emblem, Mega Man, Resident Evil, Sakura Wars, Shenmue, Soulcalibur, Street Fighter, Tales of, Tekken, Virtua Fighter and Xenoblade. Smash Bros must make sure to do better.
So, all new Fighters should be from different "new to the series" third-party licenses.
However, license and company ownership also is a matter of business and oportunity.
Nintendo knows how to take new license. Ex: Tose shares the copyright of Legendary Starfy series with Nintendo.
Nintendo knows ho to take new compagny. Ex: Monolith Soft gradually has been bought by Nintendo since the retirement of the founder of Namco. Then Tetsuya Takahashi (Front Mission, Final Fantasy IV, Romancing SaGa, Final Fantasy V, Secret of Mana, Final Fantasy VI, Chrono Trigger, Seiken Densetsu 3, Xenogears) with other former Square employees created Xenoblade. In a way, Shulk is the 1st spiritual Squenix rep.
A third-party company can have second-party licenses. Ex: Professor Layton from Level 5.
So, Smash Bros could have a new first-party Fighter that wouldn't already exist in Spirit. It could be a character whose copyright would be bought by Nintendo.