I think the very first mission in this game where you take control of Sam on behalf of BRIDGES will be delivering a corpse from ____ Knot City to Capital Knot City. After that, the scene in the BRIEFING trailer will occur. Considering you just have to handle a corpse, it gives the player an opportunity to learn about the balance mechanics without overwhelming them with a bunch of cargo to manage.
I do wonder if it's Bridget's corpse though. The scene in the E3 2018 trailer where Sam's handling a corpse doesn't have Bridget's headscarf on its right but when you see the corpse in the Release Date Trailer (no idea if it's the same corpse), you see Bridget's headscarf on it.
Sam's got the same clothing on in both of these scenes so I'm wondering if you do have to deliver her corpse or if the two scenes are completely unrelated. I'm assuming Bridget is stationed in Capital Knot City in the first place so I don't understand why you would have to deliver her corpse (unless you have to go from Capital Knot City TO somewhere else).
But yeah, that's my thoughts. I think the game will start off with a big meaty Kojima cutscene, we'll probably control Sam for a bit in a scripted sequence possibly set before the Death Stranding event to give context to this new world, but the first delivery mission we'll embark on will probably be delivering a corpse. I just think it's a good way to introduce the game's mechanics without complicating things, there's only one single thing to manage after all.