Don't forget War Machine and Falcon.
It is unclear if they start there or that's part of a major convergence where everyone eventually shows up there.
Also, there's an interesting theory floating around that it isn't Tony Stark in the Hulkbuster armor, but
Banner. The theory goes that Banner was afraid of turning permanently into the Hulk in Thor Ragnarok, something definitely reverts him to Banner form when he crashes into the Sanctum, and Banner decides to work on improving the Hulkbuster armor so he doesn't have to turn back into the Hulk and still be useful to the Avengers. There's even a scene in the trailer with Banner and Black Widow where it seems like he might have been working on a Hulkbuster component (an arm). There's also the now regular occurrence of a LEGO set hinting at this, although LEGO set toys are only batting 50/50 on this kind of leak.
If this theory is true, it obviously doesn't work for long.
Why I think that scene is near the end is because War Machine and Falcon were on opposite sides of Civil War: technically Falcon is an outlaw, and War Machine is technically still operating under the Sokovia Accords. I think they are going to keep the groups separate initially and only merge them closer to the end of the movie as they "reassemble" the Avengers to face Thanos. The exception: Vision and Scarlet Witch.
New idea #1:
I think there's more to retrieving the Mind Stone than just ripping it off of Vision's head. They tease that in the trailer, and that suggests to me there is more to that scene than what we saw. Maybe Scarlet Witch interrupts that attack, maybe the Mind Stone doesn't want to leave Vision's head. But I think Marvel generally tries to tease with misdirection, or show one scene out of context to keep audiences guessing. So if they are showing the Mind Stone being attacked, maybe that's not the moment when it is seized.
New idea #2:
What if everything we are seeing is based around the location of the Infinity Stones and Thanos trying to acquire them. Under this theory, the big ship is in New York because that's where the Time Stone is i.e. Strange's Sanctum. The scenes with Vision are because he is the location of the Mind Stone. The scene on the Asgardian ship is because Loki has the Space Stone. That could mean the alien planet is actually Nova Prime and it may have been devastated by the Power Stone or the fight to get it. We don't see the Reality Stone which is with the Collector. But that would lend even more credence to the notion that the Soul Stone is in Wakanda, because every other location in the trailer is connected to a specific Infinity Stone.
I find it interesting that the two Stones we either don't (yet) know the location of or don't see the location of are the Reality Stone and the Soul Stone. That's interesting because those two Infinity Stones are connected to life and death (the Soul Stone) and the power to remake reality (the Reality Stone). You could say that these two stones are the "reboot" stones. They could have the power to bring the dead back to life and undo any damage to the universe created by the other stones. If you want to be able to kill off the Avengers but have the ability to bring some of them back, the Soul Stone is the way to do it. And if you want Thanos to be able to wreck havoc with the entire cosmos but have the ability to return it to some semblance of normalcy later, the Reality Stone would be the way to do that.
Maybe the climax of Infinity War is Thanos defeating the Avengers and acquiring the Soul Stone, and the Part 4 movie will center around the surviving heroes seeking out the final Reality Stone as part of a plan to defeat Thanos.