I have a little hope in this actually, or rather we may see extended support beyond the fighters pass. Why?
Because of DragonBall Xenoverse 2, another game made by Bamco. That game has gone above and beyond the currently lineup of support, and still somehow manages to develop and add characters constantly EVEN NOW, nearly 4 years after the game was released. Granted, it's only a few characters per DLC, but even at the slowed progression of it, the game still feels fresh over time because of it.
I actually feel like they may go this route with Ultimate, where every year we get 1-2 addition characters and even echo fighters added via updates or paid DLC separate from the fighters pass.
We'll see though.