I usually don’t love artificially expanding scope (looking at you Hobbit) but for this one I actually would have been on board with it. Ideally they would film them all at once if possible.
In Part 1, start with a Thanos, Thor, GotG and Collector story that focuses on Thanos’s backstory plus him getting the Space, Reality and Soul stones and killing Loki/Gamora. Gamora’s death is the climax with Strange/Stark/Spidey waiting in ambush for Thanos on Titan as cliffhanger.
Then Part 2 would be Thanos wiping out the Nova Corps to get the Power stone in the cold open, fighting Strange/Stark on Titan and taking the Time stone, before heading to Earth to smack down Wakanda and take the Mind stone.
Part 3…is whatever it is. That when you finally get everyone on screen at once.
Disney likes money, so I can only imagine that the issue was contracts with the big actors, but they could have basically omitted Cap, T’Challa, Black Widow, Bucky and maybe Vision/Scarlett Witch from Part 1. Then Loki & Gamora are dead in Part 2 and maybe you downplay the GotG’s involvement on Titan. Strange, Stark, Spidey and maybe Thor would have to do quite a bit of lifting in both movies I suppose, but collectively it would only amount to 1 movie’s worth of call times.
Maybe something like this:
Part 1 - Act 1: Thanos attacks and wipes out the Asgardians. Black Order descends on earth and tries to take the Time Stone.
Part 1 - Act 2: Thanos backstory. GotG and Thor meetup. Ebony Maw dies. Thanos beats the Collector.
Part 1 - Act 3: Gamora dies, Thanos gets the Soul Stone. Strange/Stark/Spidey set up shop on Titan.
Part 2 - Act 1: Black Order lays siege to Wakanda. Thanos destroys Xandar and gets the Power Stone. Thor tries to make Stormbreaker.
Part 2 - Act 2: Thanos gets jumped but eventually wins the Time Stone. Black Order breaks through at Wakanda.
Part 2 - Act 3: Thor is revived and appears on Earth to rout the Black Order. Thanos shows up, lays waste, kills Vision and takes the Mind Stone. Snap.
This gives us a chance to see what happened on Xandar and Knowhere which is nice. The battle of Wakanda has time to breathe a bit. Thanos gets a full backstory and a motivation that maybe makes a little more sense. We see Thanos get all the infinity stones and we can show each one’s power up. We have time to internalize Gamora’s death before the Snap™. Perhaps the biggest weakness to this is that Part 2 - Act 3 might be a little light unless Thanos decides to drag the fight out. You could move the meat of the Battle of Wakanda to Act 3 and extend the battle on Titan or something, but there’s plenty of stuff there. Maybe the first Hulk/Thanos fight happens in Act 3.
Anyways…good idea.
BTW, I haven’t noticed it mentioned here yet but Adam Warlock is definitely making an appearance in the finale. Somehow he and Captain Marvel must team up, maybe that explains where Captain Marvel has been for the last 20 years.