I have read spoilers of how he comes back etc... what it think would have been great, is if in the first WW, we have a scene, towards the end, before he sacrifices himself, where Steve is missing for a while, then a flash appears, light lightning and Steve emerges from the background, looking solum. Diana asks if he is ok as he seems distant, he apologizes, kisses her and says he loves her, which shocks Diana and I guess the viewer, maybe he just realized something, who knows... but the movie continues and he sacrifices himself.
No one thinks anything of it, maybe just a plot device to make him realize what he has. Then we watch the sequel and learn that the villain takes him out of his reality, into the present, for Diana - she doesn't know and Steve explains that he was just literally on a battlefield with her.... but if he doesn't go back, history is rewritten... and could change everything.