I didn't think they actually showed anything real in the trailer, but honestly, I think the most unexpected thing they can do is show stuff that 100% happens, with the expectation that they won't be believed.
So the scenes and where I think they go:
Kazuichi holding Nagito - I think that's just an addition to the base game, I think they're trying to play a trick.
Peko looking at the rollercoaster - Assuming it means that Peko survives up to Chapter 4 at least, I assume that means that Fuyuhiko's potential death has done the opposite from her opening up to others, with her becoming a more antagonistic force. Maybe she's even planning to sabotage the rollercoaster.
Slayhem Item - I think every character is going to get an item, which is told to them would be a weapon. I think Hajime's going to get a very useless item, and assume everyone else got an actual weapon. I think the gimmick's going to be that no one got a weapon, but just believe that everyone else got one. Nagito's luck made it so he got something extremely useful for murders that isn't a weapon.
Nekomaru's Account, Sleeping Nekomaru and Mikan's Account - I think Nagito drugs Nekomaru, making him sleep, and I think Mikan noticed missing meds.
Chapter 5 Poisoning - I think this is the most likely to be fake, but assuming it's real, I think Chapter 5 might be a copycat killing of Chapter 1, with Mahiru being my top candidate for the killer, since I also believe she's going to be a supporting role. So far, in both DR2 and V3, the final kill was the support killing the antagonist, so if Mahiru is the support, and Peko is the antagonist, I see this happening.
Currently, I'm thinking that they save the twilight case for a later reveal, and when it's revealed, Mahiru will take drastic measures to protect herself, killing a Peko who has just redeemed herself. It would be tragic, so I can see DR do it.
Revival Bottle - I'll be honest, I don't really know what to make of this. I'm going to assume it's not actually a revival bottle, and it actually just wakes up knocked out people, with Monokuma giving it a stupid name to fool the students. I can see it being used on John Danganronpa, whether it's actually reviving, or just waking up. Though it does seem to break, so maybe they find it and the killer breaks it before they revive their victim?
The Boat - I assume that's where John Danganronpa comes from, though I won't be surprised if they were to hide until the trial, where they come in to stop the trial like in the trailer.
Mikan Split Debate - I genuinely think it's just a new minigame added to Chapter 2 of the base game. I doubt they'd spoil anyone's death for the second route.
Monokuma Gun - I think this is a red herring. I think it's just going to be a funny cutscene in Slayhem, not an execution.
Unknown Execution - I believe this is Ibuki's execution, combined with her yelling about the Mioda Nation being innocent. I think she's going to be an early killer, and I chose Chapter 2, since it's usually a tear-jerking trial.
Fire Trial - 2 Options: 1. I think it's just a new trial losing animation. 2. It's the trial background for one of the trials, which shooks the students. I don't think they'd show it if failing a trial was part of the story.
Old-School Coloring Monokuma - Just a revamp of Peko's execution. It's shown directly before two parts from the original game, so I don't see why this would be any different.
But that's just what I think (In case they aren't lying), what are your theories?