By virtue of being the final slot, I can see that final slot being reserved for the new horizons of Nintendo, and Sakurai has specifically employed this method before with Pokémon. Might be the downfall of Sword and Shield (Since it was revealed while Fighters Pass 2 was decided, so it didn’t need to be the last character for that to work), but could work for Arceus (though the timing doesn’t mesh as well as it did before, and Sakurai might believe that a 2022 release might be a bit iffy to release a character for in 2021)