There was one very strange moment in the first part of Crisis on Earth-X last night that has a lot of people scratching their heads. Just prior to the start of the wedding, a caterer comes up to Barry Allen (Grant Gustin) and talks to him about how excited she is to be at the wedding, and how he must be feeling. She offers him a glass of water and touches his arm in a sign of encouragement. Again, all very strange for just a random waitress and not something the writers of the show would do for no reason.
The caterer was played by Jessica Parker Kennedy from the Secret Circle, 90210, and Black Sails. Those are pretty big credits for that part, unless it means something more. This is the type of thing that on most shows would seem like something that would pay off later in the same episode or in this case, during the event. But with the Arrowverse shows, it really could be setting something up for the long term. And with a universe that includes time-traveling and alternate Earths, she could be just about anyone from anywhere and anywhen. (Anywhen should totally be a word with all the time-traveling shows on the air now.)
That's not to say I don't have a guess on who it is. With how personable she was with Barry and how excited… my mind immediately went to either Dawn Allen or Jenni Ognats. Dawn in the comics is the daughter of Barry and Iris, but born in the 30th century along with her twin brother Don. They were born shortly before Barry lost his life in Crisis on Infinite Earths and grew up not knowing her father. Though they both were born with some speed powers, they didn't use them until they met and helped Wally West. They became known as the Tornado Twins. They would eventually run afoul of Professor Zoom, getting trapped on Earth-247 and would later die along with Don defending the Earth from the Dominators. She was created by Jim Shooter and Win Mortimer.
I'm more inclined towards Jenni Ognats, Dawn Allen's daughter and Barry and Iris's granddaughter. Jenni is also the speedster known as XS and a member of the Legion of Super-Heroes. She was created by Tom McCraw, Mark Waid, and Jeff Moy in the pages of Legionnaires #0. With the fact this was an episode of Supergirl and that they're going to be exploring the Legion this season, it would seem a likely Easter egg, or even better — she might be one of the members still in the tubes on the ship that Mon-El and Saturn Girl were on. So, my money is on her being XS.