You know, you joke, but there's a potentially good story in the idea of the Flash rescuing Steve, ensuring that he doesn't die in WWI. . . and that doing so changes nothing substantial about the world. Partly because Barry wasn't stupid enough to drop Steve off in the middle of the still ongoing god-battle, so Steve's survival gets to be a surprise *after* victory is achieved. . . and partly because the world does not depend on Steve Trevor's death. Diana did not need his death to become the heroine she became, and his continued life does not produce some inexorable paradox.
So, Barry returns to the present, and its the same. Except Diana has additional fond memories of further years with Steve, and there might be a few grand children running around the world. Diana thanks Barry for the thoughtfulness, but does inquire whether this was what he expected.