Infinity War is gonna clear $2 Billion -- which is awesome.
I can't wait for next May and the second installment.
The Russo brothers have alluded to 'mysterious' forces at work in Part 2 and all the Marvel team have denied the more obvious time travel type speculations for bringing characters back to life, so I can't hazard a guess as to plot, but I do have a few guesses about how the film starts and ends.
Interestingly, word is that Kenneth Branagh voiced the emergency distress call that starts the film (which I particularly liked since it reminded me of the tanker mid-battle radio broadcasts at the beginning of Fury).
Branagh directed the original Thor back in 2011, and his continued involvement makes me think that he may have an acting role in Infinity War Part 2 (or whatever it is ultimately called) as the new civilian leader of the Asgardian survivors.
One way to start the film would be for more ships coming into the earth's atmosphere. The surviving Avengers (including folks from Wakanda) scramble to meet the new threat -- but it turns out to be a false alarm. Turns out, the arriving ships are the Asgardian survivors from Thanos's attack which we saw at the beginning of Infinity War (during Infinity War, Thor says that Thanos killed half his people -- so the second half, the survivors, are likely with Valkyrie and Branagh's character, who take them to earth).
Meanwhile, Tony Stark and Nebula find a way to get back to earth -- but only after they have to collaborate to science-the-sh*t out (ala The Martian) of the wreckage of ships on Titan to come up with something serviceable to get back to earth. For me, the Stark/Nebula pairing is the most bizarre pairing there could be in the MCU -- beyond even Rocket and Bucky Barnes. Upon returning to earth, Stark learns from Happy Gilmore that Pepper Potts turned to ash.
As to the climax, I like one recent fan theory: at the critical moment, Steve Rogers takes some suicidal action to buy Stark time -- and Stark tries to stop him by saying 'stick to the plan' and Rogers replies 'this was always the plan' -- echoing Yinson's scene from the first Iron Man.
As I said above, I have no idea where the action or story goes from here but I suspect the film ends with a wedding -- between Stark and Potts (who is somehow restored) and maybe Gamorra and Quill. It would be great if Stan Lee officiates the wedding and there are many mid-credit scenes of hilarity at the wedding -- which I suggest would be in Norway at the site of the new Asgardian City that Odin picked out in Ragnorak.