With the other post getting over-flooded with replies, it's time to start a new one.
It's simple, where do you see the series going after that "meh" bomb of Second Son? Well with all that stuff that happened around E3, the "announcement" of Sucker Punch working on a game, I'd really love to see Nate Fox pull a Sly 3 out of this one. Have it cap the trilogy(yeah I know Second Son is a "main game" but it comes off as an Iron Man 2 to InFamous 2, you know?). Now that doesn't mean that it has to be the last game in the franchise. Have it end the overarching story. Then sequels could go the route of Sly 4(or for you XBOX people, the Halo franchise. Halo 1-3 was it's own self-contained story, then Halo 4 - 6 is it's own contained trilogy).
Oh and...if we have to have Delsin in the sequel, fine. But everything afterword. No, no more Troy Baker. He's taken over enough franchises as-is >_>.