Losing Ironman and Vision alone wouldn’t be enough emotional weight for such a grand climax in my opinion. The Guardians would likely lose Drax too; This one that he is on a suicide mission to fulfill his title as Drax: The Destroyer by killing Thanos. In fact, I think all the heroes will die horrible deaths first, then someone travels back in time to change things (possibly Dr Strange) and then they comeback for a second final showdown, but the heroes that die in the second showdown stay dead.