I have heard a rumor about what will happen to the avengers in the run up to "Avengers : Infinity War" Now this is a rumour, From latinoreview, While alot of there reports are usually false This one seems highly plausible. You have been warned on its believability and it may be a spoiler so its not safe if you don't want to risk possibly spoiling it.
towards the end of Captain America: Civil War, it seems Steve Rogers will indeed be killed off as seen in the climax of the Civil War comic story arc "Death of Captain America". It will be at the hands of Crossbones and will be during the feud between Rogers and Tony Stark after Stark finds out it was Bucky who killed his parents.
The sources go onto claim somehow Iron Man will end up in space but are extremely vague on this. Remember Tony Stark does join the Guardians of the Galaxy in the comics so this might be how the Guardians warn The Avengers about Thanos since remember...they have no idea he even exists!!
Lastly, in Thor: Ragnarok many Asgardians will die and this will lead to Thor (Chris Hemsworth) being sent to a "Galactic Prison" of sorts. Is this him finally being sent to Valhalla? And just like that the top three Marvel characters will be out of Avengers: Infinity War Part I.
Who will then defend Earth from Thanos? Apparently the "New" Avengers may end up being lead by Doctor Strange (Benedict Cumberbatch) himself along with Black Panther (Chadwick Boseman). Others on the team include Scarlet Witch & Quicksilver (Elizabeth Olsen & Aaron Taylor-Johnson), Ant-Man (Paul Rudd), The Wasp (Evangeline Lilly), Daredevil (Charlie Cox), Luke Cage (Mike Colter) Jessica Jones (Krysten Ritter) and Iron Fist (TBA). New additions also include Captain Marvel (TBA) and Spider-Man (TBA) who will also play a large part.
No sign of Hulk, Hawkeye or Black Widow. However it is to be assumed that Bucky would take the role of the new Captain America.
Also, any of the Guardians of the Galaxy and Inhumans could also play a part in the battle.