Like Coulson said all the way back in The Avengers, "This was never gonna work if they didn't have something to..." and then he died, but his point lives: The Avengers need something (or someone) to avenge. A major death in their own ranks would bind them for the foreseeable future. It's just a matter of whom.
As pissed as I am that we'd never see his wedding, Tony Stark could definitely die. Infinity War made it clear that not even Pepper can get him to take the Iron Man suit off. The chances of Tony ever retiring are slim to none. Plus, his protégé Peter Parker is just beginning his solo movie career. Spider-Man would benefit narratively from losing yet another father figure and his tech benefactor. However, Thanos defeated Tony already and almost killed him, which is why my money's on Steve Rogers.
Steve had a pretty small role in this movie, but he's primed to take the lead in Avengers 4 as one of the six surviving members of the team. He has a Captain America heir apparent in Bucky, who's been to Spa Wakanda and even referred to himself as a "semi-stable man" in the movie. Progress!
Most importantly, Steve hasn't really faced Thanos yet. During the final battle in Wakanda, the only Avenger who briefly holds Thanos up is good ol' Steve Rogers. Thanos looks at him with some kind of uncharacteristic emotion — maybe confusion at Cap's unexpected strength or maybe (hopefully) fear.
Either way, as sad as you are now, think about how sad you'll be next year when someone actually dies.