The problem is that most of the memorable PS1/PS2 games on our side(s) of the world weren't coming from Sony.
That's not a problem at all, as they could always get some third parties on board. Even Nintendo did that themselves with Brawl, so there should be no reason for Sony to not use 3rd party characters. And if that Long Live Play commerical truly is anything to go by, then there should definitely be some 3rd party characters present.
I'd be quite surprised if Snake and a Final Fantasy rep aren't in this game.