It seems like it might involve time travel. That's the simplest explanation for Pine's character returning. Perhaps, instead of killing him, the explosion somehow sent him forward in time to 1984, where Wonder Women discovers him after living the last 66 years grieving over his death. Whatever the explanation, it appears as though it'll partially play out like a time travel movie with Pine's character being a fish out of water in a 1984 that's futuristic and strange to him. I suspect that a lot of the movie's humor will revolve around that. It could be funny to see a character from 1918 react with wonder and confusion to 1984 technologies and fashion that are now terribly old-fashioned and obsolete to us.