It can ONLY be Hopper surely - Joyce and Murray survived so there was no other American in the immediate vicinity who was unaccounted for. And his body was never shown, unlike the Russian scientists when the device blew up right at the start of episode 1.
I just think he’s too big a character for the writers to do a ‘Game of Thrones’ and kill him off ...
Well the fact the russians know all about this "upside down" and how to get into it suggests they got information from someone, 11's father was meant to have been killed in Vietnam so maybe they somehow got a hold of him and learned about it. It could very well be Hopper as we never seen him die, waiting game to find out.