Together Again

This is a pretty weird story idea but I can't get it out of my head. P.S In this story Mon-El worked it out with Imra to get a divorce and came back to the past to be with Kara. Also Savitar in season 3 of the Flash hasn't happened yet.
Mon-El just got back from fighting brainiac in the future. Him and Kara just got back together when Barry brings heartbreaking news. What is this news? Will it affect their happiness?