I think Steve for sure will be going home, I just don't know if it's in Infinity War or Avengers 4/The Film Formerly Known as Marvel's The Avengers: Infinity War - Part 2. Chris Evans' contract is about up, plus his death would thematically fit for the end of this MCU Avengers lineup (old school team leader who doesn't fit in the post-war world sacrificing himself, motivating one of his friends - Bucky or Sam - to take up his mantle).
Tony would be second; it again would signal the end of an era, paving the way in an almost meta sense for the audience to move on to the new Avengers lineup, as RDJ Iron Man has been the face of the MCU since his kick-off solo film a decade ago. Also, he could sacrifice himself to save the world and his comrades, something that would show how his character went from a self-centered, egotistical playboy to a someone willing to lay down his life for the world's safety.
Thor is also on my list, as his character and movies have been more about explaining the cosmic side of the MCU and pushing it forward (he was a walking plot device in Age of Ultron, Dark World introduced Aether, the Collector, and the fact that multiple Infinity Stones were being sought, Ragnarok mid-credits involved Thanos confronting the Asgardian refugee spaceship). Once Thanos is gone, Thor - and Asgard for that matter - might go with him.
On a side note: it was a missed opportunity to not kill off War Machine in Civil War. Could've really raised the stakes for not only that movie, but also going into IW as all the heroes have to band back together after their skirmish being responsible for the death of Rhodey. Just an opinion.
Haven't seen the latest trailer. Does it seem to spoil too much? Like, is it a BvS or Terminator: Genisys 2nd trailer? Those two pretty much eliminated the need to see the movies.
Based on some comments from various sites, I wouldn't be surprised if there are multiple casualties in Infinity War (though how many would be permanent is anyone's guess). But that's just going by spoiler-ish comments some of the actors made, like Mark Ruffalo, who said that Infinity War does not end well for the heroes (and considering the backlash he got from co-stars and the like, it seems legit), and other comments from the studio, who apparently said that a lot of the characters that feature in 3 won't feature as much in 4. So, this, to me, suggests that more than one person is going to bite it.
__________________ Never take life seriously. Nobody gets out alive anyway.