So here's the first official trailer:

It's a lot to unpack. There are scenes in space, NYC and Wakanda, meaning that Black Panther is going to likely be the direct set-up movie for this one. Bucky's back. Cap's gone dark and beardy, Widow's gone blonde. No Hawkeye in the trailer, but he's in the team shots from Vanity Fair.

Looks like Thanos arrives with the gauntlet and no stones - or at most the power stone (the purple one from the first guardian's movie, which wouldn't bode well for Nova Corps) - as he's shown slipping the Tesseract / blue stone into place and it's only the second stone in his collection.

Going from the leaked opening with Thor landing on the Guardians' "Milano" ship, and the after credits scene from Ragnarok where a ship (likely Thanos' Sanctuary) overtakes the Asgardian refugees' ship, and the fact that there's plenty of footage of Bruce Banner / Hulk in this trailer, I think the Loki scene is a misdirection. He obviously took the Tesseract at the end of Ragnarok, no surprise there, and Sanctuary tracked it down. But rather than the apparent surrender of the Tesseract to Thanos that the trailer seems to show, I think what we'll get is Thor trying to be heroic and being ejected from the Asgardian ship, then Loki stepping up and using the Tesseract to transport the survivors (those are Asgardian-dressed bodies he's stepping over), including himself, Valkyrie, and Bruce back to Earth.

It's likely from this teleportation that Bruce / Hulk falls through the Sanctum Santorum, which is where the naked Bruce is in the basement with Strange and Wong looking down on him, perplexed. I doubt that Loki could simply walk into the Sanctum after his first encounter with Strange, so Bruce may have been the only way to get a foot in the door.

They show Thor in a ring similar to the portal/stargate that appears over New York, so maybe that's how the Guardians get there.

I have to wonder if Peter's going to out himself as Spidey by leaving the bus directly - which could be cool.

Obviously things aren't going to go well with Vision, seeing as he's got the mind stone embedded in his forehead, but it looks like he and Wanda are at least going to get a moment or two in this one. (unless that's an illusion) And frankly, I think seeing a truly pissed-off Scarlet Witch in the aftermath of watching her sweetie ripped apart would be something to see. (remember, this is the woman who - in the comics - more than decimated the mutant population with three words out of grief for her child)

My big question is whether the final gem is with Heimdall, making it the source of his "sight" and oddly colored eyes, or if it's in Wakanda, making it the reason Thanos' forces amass to attack that particular country.

I'd have to guess that this one will end with Thanos assembling the gauntlet in full. (I'm curious about the show-runners describing this as Thanos' story / an anti-hero story). We'll likely see the death or near-death of a few major players. The woman who plays Hawkeye's wife is in this one, so a funeral or hospital scene wouldn't be out of the question. Karen Gillan (Nebula) says she's only in one scene for a short while, so she's a likely candidate for death. The obvious choices for martyrdom are Iron Man and Captain America, though I think they'd kill Pepper off before Tony to tick him off and make him stupid (likewise, a bad injury to Spidey would make him reckless), however the kid who was in Iron Man 3 is also in this one, which brings up another possible funeral scenario. I'm pretty sure Cap's a gonner, but maybe not until Avengers 4, leaving Bucky or Falcon to pick up the shield. Ant-Man and Wasp have a sequel, so neither of them will die. I can't see them killing the Widow, and I doubt Thor's in danger. Loki, maybe, as the bow on a redemption arc. Valkyrie almost certainly, as she wants to atone for the deaths of her fellow-riders in battle. No way will they take out Strange or the Hulk, but I'm not sure about Wong.

One notable character who's MIA is Hela from Ragnarok, suspected of being Thanos' "Lady Death" in the MCU. Maybe they're holding her presence for effect. And I think it would be hilarious if Phil Coulson finally got his "back-from-the-dead" moment in the last minute or so of this one. He just walks in and asks "What did I miss while I was in Tahiti?"

Basically, all I want out of this one is one scene of Tony and Rocket in full-on engineering mode, shooting one-liners at each other and being brilliant. And one scene of Gamora, Widow, and Valkyrie kicking butt. Give me those, and I'm a happy nerdling.