Technically now, since that was only a two month gap, that means we might have a longer wait on at least one fighter next year since three are supposed to release throughout 2021.
It seems though like characters with simpler designs and such have smaller gaps. Min Min and Steve probably took a while since they were unique, im sure Sephiroth will be fun and all but you can use other fighters as bases.
So I think its safe to expect at least one complex or gimmicky character, Rayman would fit the criteria because of his unique design, for example.
Also, this is just one more theory of mine, but if this was a two month gap and they continue the two month gaps like Steve to Sephiroth, then its possible we get our next fighter revealed around late February to early March, which would line up perfectly with Pokemons 25th anniversary!
Plus, its also worth pointing out besides Sephiroth, we've recently had Smash reveals happen the same week as Pokemon news
We had a Pokemon Direct like a week before Byleth was revealed
Min Min was revealed two days before the hideous Pokemon Unite presentation (oh uhh also Cinderace is coming to Unite)
Steve was revealed two days after we got Crown Tundra information
So its very possible Fighter 9 is revealed whenever we have Pokémon news next, which will probably be around late February