There is one important thing to take from Seph's inclusion (besides the fact that second unique fighters are now possible for third-party series): so far, everyone in Fighters Pass 2 is looking to be a character or concept that was planned to be included earlier, but had to be moved to DLC.
- filled the void left by Spring Man not being able to get in base game as a fighter representing ARMS.
- 's negotiations occurred shortly after 4's content support completion, with him missing out on FP1 and filling in for him in the meantime.
Now we get to Seph, who was in the infamous Seven Squares list, which is two years old, and it's rumored that he was going to be all the way back in 4 before those plans fell through. The most telltale thing heralding his eventual arrival is
's associated content being oddly minimal even after coming back for the base game of Ultimate.
So what I'm getting at here is that some (if not one) of the remaining fighters are likely to have been planned to begin with, by having minor content placed in the base game as a holdover until negotiations for a playable appearance are complete.