Wally isn't doing much... Legends could do with a speedster.
Here's a thought -- there have been two beats lately showing Curtis/Mister Terrific geeking out over the Waverider, first in his cameo in "Return of the Mack" and then in "Crisis on Earth-X, Part 4." In the latter case, Zari and Amaya even had an exchange about how they'd "picked the right guy" -- in context, the right guy to talk to the Ray about the Reichsmen's doomsday weapon, but maybe there was a subtext there. Could they possibly be laying the groundwork for Curtis to join the Legends?