I think this is an opportunity for them to start off on a fresh leaf and take the marvel universe in a different direction from the comic books. I don't really know how successful the comic books are. But nothing was ever so striking that a casual fan like me ever sat up and took note.
So, I believe the average movie goer really has not much of an idea of how the story goes after Thanos. At the same time, it would defy belief that they fight a villian like Thanos and walk out of it with zero casualties. So, use the way things are tilting now to forge a new path.
Wakanda is hot, black panther is hot. Spidey still has its fans. Surprisingly, Iron man as well. Bucky is interesting, but NOT the one I would have in mind to lead the avengers if you take out Cap. He has no moral authority and has shown zero leadership abilities so far. And its not like on a power level, he is the best either. So why would the rest of the avengers even listen to him, much less let him lead them?
If it was me, I would have captain america die. He is the kind of character that would make for a glorious moving dying scene, killed by Thanos. I would even have Bucky the winter sholdier die as well. He is like a muscle man. He just fights and is a plot device for conflict between Cap and iron man. Once Cap dies, Bucky loses even that.
I honestly think Vision isn't very interesting unless you want to make him and scarlet witch a thing in the new team. I actually think the average movie goer would not miss him too much if he dies. Same for scarlet witch actually. Yes, she is powerful, but I don't think the average movie goer remembers much of her.
I would say War Machine makes an interesting replacement of Ironman if we really want to have Iron Man die. Although its a pity because the man behind Ironman is what makes ironman interesting, not so much the suit. And if we kill off Cap, then we will always need an ernest patriotic do gooder sort of person which fits war machine to a tee.
Black Panther will take on the leadership role. He is a king already and Wakanda has the technology. And he is already a massive fan favorite. Black panther also opens the way for integration and cross over with Xmen in future because he and Storm are together next time.
I am actually in favour of black widow surviving. There are many heroes with big powerful abilities, but her kind of spy skillset and street smarts is less. Her surviving allows the MCU to explore other types of genres like spy, etc in future as well. And well, not everything needs to be hammered in with a blunt force hammer as a solution. And if the MCU ever gets polarised because of the politics in it (because you are introducing mutants next time), then black widow plays a very useful role in the espionage role. I think she has far more she can contribute over more powerful superheroes. Besides, its always good to have women in representation.
I keep thinking whether Strange should stay or not. The problem is his magical powers then to be too powerful or too weak. There isn't a middle ground. Either they are useless against an enemy or they are too strong. And while he did get a solo movie, he wasn't exactly a resounding success in terms of reception.
I think Thor should survive in one fashion or another. With Asgard gone now, Thor is the only key remaining link to Asgardians and their mythology. And Thor ragnarok was so well received. I think Loki should survive too. He is just too interesting a character who at any time could morph into a villian to just kill off.
And Spidey is so well entrenched I think he should of course stay too. I think guardians are just making a cameo appearance so I don't expect any of the guardians to really die. They are a team in their own right, so if any of them ever die, it will be in their own sequals and not in infinity war.
I am fine with Falcon dying too actually. he flies, big whoop, tons of superheroes flies these days. He wasn't interesting when they introduced him, and unless he gets a solo movie of his own, I don't expect him to be interesting.
So, yeah, for me, I would be ok with Captain, Bucky, Falcon, Vision, Scarlet Witch, and maybe Ironman dying too. Then black panther should lead the team after cap goes. Actually, I feel that if RDJ's salary is not an issue, he should stay on, because RDJ is just that good at Tony Stark. From a character stand point, RDJ is tons more interesting than almost all of the rest of the superheroes. Take away the suit and the powers, and most of the superheroes fade away to nothing. Take away the Ironman suit, and you still have a bigger than life RDJ.
Actually, I am looking forward to the merging of the xmen and Avengers universe. Because I think in terms of villains, Xmen has a lot of very memorable Villains, the avengers, not so much. And right now, we have loads of cool superheroes, we don't have enough cool villains.