It doesn't have to be "Waverider" himself but make a movie that's a riff on Armageddon 2001.
Waverider is from a bad future, where a hero turned villain has won... he travels through time to find the secret identity of this villain and warn the heroes early/stop this from occurring. Waverider can see the timelines of peoples lives when he's close to them so what we get is scenes from each heroes future and a lot of the futures are bad.
Then it occurs to me this is similar to the plot of deadpool 2.
Damn.
DCEU needs a much bigger universe I can't just watch the justice league win fights over and over again. Whats their best story arc? Make that movie.