There's a certain character that comes from the PS1 era. Very popular. So popular that his inclusion would've generated more hype and more sales for this game. (Maybe preventing the Great Fall of Superbot?) Crash Bandicoot was beloved by a lot of PS fans but not even he can top the fame of HIM. The roster could've been more diverse with an RPG rep, and this guy is from one of the best-selling RPG franchise in the world. He's the man with the giant sword and the motorcycle.
I am not a fan of him at all and the game he's from...but still, he would have attracted a s*** load of people to PSAS. If you think about, anime fans love him and there's bazillions of them out there. RPG fans (most) love him and he's a very nostalgic character like Parappa. Most players who grew up with the PS1 loves him. His fanbase is HUGE, overshadowing anyone else's in the roster. I know it's only one character I'm talking about here, but if you look at all the variables, he would have had a major impact on the sales.