My theory about the opening (spoiler tags in case I happen to be right) SPOILER: which ultimately lands Nate and Sam in jail. My theory splits in two here. Either the old guy from the trailer is their dad and he recruited them to do the museum heist in the first place (remember it’s a three man job), so all three of them end up in Turkish jail together or after Nate and Sam get thrown in jail, they'll meet that old guy in the trailers (who may or may not be their dad) and just like they used to escape from the orphanage they'll plan an escape from the prison. The final scene before the opening credits is Sam getting gut shot in order to let Nathan escape. The opening credits themselves will show a montage of everything that has happened after that point, spanning Drake's Fortune, Among Thieves and Drake's Deception, Elena and Nate’s wedding(s?). After the credits, we start the game proper with the scene of Nate stamping papers until he hears a knock at the door. “Sigh. We’re not open yet!” he calls…")

This has all come from inside my brain, and various musings around the interwebs. I actually hope I'm wrong and I didn't just spoil the opening for myself based on pure speculation.