Before I start I wanted to say I'm sorry for my bad english =). As we all know the third instalment of the avengers and the conclution of a lot of what marvel have been building up in the mcu will come with the two part infinity war (with black panther and captain marvel movie in between this two part "finale"). All of what happened and will happen in the mcu in the next couple of years will lead up to this big event which will finalize a great part of the mcu on may of 2019.
So to be brief here are my predicitons on what will happen on Infinity War.
A New Team will lead the fight on the first part
After the events in [The Avengers: Age Of Ultron](movie:293035) and [Captain America: Civil War](movie:994409) the original team will dissassembled and a new team will take the mantle of the world greatest heroes. I belive that by the beginning of Infinity war Captain America will in fact be dead after civil war, hulk will be in space, thor will be incapacitated and iron will be out of reach. This new avengers may be conformed by a lot of heroes and the may be:
-The new Captain America (Bucky Barnes)
-The Defenders (Daredevil, Jessica Jones, Luke Cage and Iron Fist)
-Some of the original team members (Black widow, hawkeye and after AoU Scarlet witch and vision)
Spider-man and Dr. Strange will play a pivotal role
After the most known avengers are out of the picture by the time infinity wars hits theaters your friendly neighbourhood spider-man will take a lead role leading this new team against thanos and his team. I think spider-man will begin as one more of the team but by the middle of the movie will take the leading role. This one may be a safe bet for marvel and the fans, with all of the most beloved members out of reach its up for spidey to take a crucial part in this movie.
Another avenger that may have a vital part in this story is Dr. Strange, being involved in the arts of magic Stephen Strange will be a key part against Thanos and the infinity stones he may have.
Thanos will collect all infinity stones by the end of part one
By the time the first infinty war premiere thanos will already have the infinity gauntlet but he may not have all of the infinity stones. Some of which we all know like the orb (Guardians of the Galaxy), the scepter (avengers), the tesseract (the first avenger and avengers), the aether (the Dark World) and some of which will be presented in the next mcu movies. I think one of them may be presented in Dr. Strange and the last one could be shown to us in the first infinity war. Thanos will have in his power at least 4 infinity stones by the start of this movie and by the end of the movie and after a fight with the new avengers he will get all of the infinity stones.
Not all of the new members will make it
After facing Thanos for the first time and giving him a great fight not all of the new avengers may make it. This first encounter will have some casualties which will lead to the reamining members to ask for some backup. The ones who i think will be in risk will be ant-man, some of the defenders (hopefully not daredevil) and vision. All this is not because of their lack of skills but cause of the moral hit to the new team and that could help the audience start not seeing Thanos as a normal bad guy.
But wait, by the end of part one the new team will not be hand empty they will get a way of bringing all the original and new lost members of the team back to the game, this may be through using an infinity stone (maybe the soul or the reality gem) before thanos taking control of them or by the magic of Dr. Strange. So now we have the original captain america back and other lost members. This will be sum with the fact that Iron Man will be back from wherever he was, and Thor will be rescue from the imprisonment his is rumored to be by the end of Ragnarok. So this may end part one, with the new and most of the original team assembling and getting ready for round two against Thanos. In part two and after a lot of stuff between members of the team like the forgiveness between Iron Man and Captain America :) the avengers face Thanos for the second time and this time they do it right. But in the middle of the fight guess who will join the fun, your favourite a-holes, the Guardians will join the fun with a green friend that they found in the outerspace, the one and only hulk. So now this new and massive avenger team is ready to take on Thanos.
The Stones/Gems will be the key of beating Thanos
Realizing this the avengers will focus on getting the gauntlet from Thanos and after this big, massive and awesome fight between Thanos and his army the avengers will get the stones out of the gauntlet and six avengers will take a gem pretty much in the same way guardians of the galaxy did with the orb. I think the ones that will do this are Captain America, Thor , Iron Man (yes, dispite being a normal human), Spider-man, the Defenders and the Guardians as a unit the other two gems. So with all the power in the universe the avengers will put an end to Thanos and bring peace back to the universe.