One of the things folks are talking a lot about coming out of the Crisis on Earth-X was a simple scene that might turn out to be very big. During Supergirl, a young female caterer went up and offered Barry Allen (Grant Gustin) some water, talked to him about being nervous and how excited she was to be there. It was an awkward scene where she reminds Barry to say, "I do," and touches his arm. In such a busy event, no scene was just there to be there and the fact they cast Jessica Parker Kennedy (Black Sails) as the caterer tells us it's important.
The prevailing theories are of that she is either Dawn Allen, the daughter of Barry and Iris (Candice Patton), or Jenni Ognats, their granddaughter, and the Legion of Super-Heroes character XS. For some reason, most people are counting on it being Dawn, one half of the Tornado Twins, but my money is on Jenni and there is a good reason. According to TVLine, they've been assured that we will see the character again this season. They also talk try and parse out the exact meaning to what she said and did. That they never got to the "I do," part in the ceremony and later when they got married on the lake front, John Diggle (David Ramsey) never asked them to say "I do." So they theorize that he will have an opportunity in the future to say "I do," about something else. Also, they figure her touching him will be used later for Cisco (Carlos Valdes) to vibe her. And all of that is completely possible if her future appearance is on The Flash.
But, she was introduced on Supergirl, a show that is currently dealing with the Legion of Super-Heroes and even has a ship full of them. The argument against that is Supergirl is Earth-38 and Flash is Earth-1… and the wedding was on Earth-1. Well, she's a speedster, so moving between Earth's isn't that hard. But the bigger question that no one seems to be asking is: why didn't she warn Barry about what was going to happen? If she is the daughter or granddaughter of Barry and Iris, how would she not know about Nazis popping up and killing the priest? The whole thing led to a death of a Legend and friend of Barry's… something that might've been prevented with some warning. And don't give me the, "She didn't want to mess of the time line," crap. She's an Allen — they screw up more timelines before breakfast than most people screw up things all day long. And it negates the whole, "remember to say I do," thing.
The only really good answer for her not warning him and for not getting into the fight is she isn't from Earth-1 and left before the ceremony. And looking at the storylines for the season: Flash is dealing with the Thinker and 12 new metas, while Supergirl is dealing with Reign and a group of heroes come together to stop the Worldkiller — a group that includes the Legion. Where does this character seem to fit into better?
But, like everyone else's… it's just a theory.