Here's the challenge...the Big Seven members of the Justice League each get to pick a backup, who can either fill in when the A-list hero is unavailable or show up for big end-of-the-world stuff.
Superman picks Captain Marvel . . . similar power set, and Marvel is even more of a boyscout that Supes.
Batman is going to want some more brains on this team for when he's not around. The obvious choice would be Nightwing, the original Robin, but those guys have a complicated relationship... Mr. Terrific of the JSA is the world's third-smartest man, although Dr. Midnite is another contender. I could even see him going with Sandman for his detective skills.
Wonder Woman picks Power Girl, to ensure the presence of a strong woman on the team.
Green Lantern [Hal Jordan] goes with Green Lantern [John Stewart], because the Green Lanterns have this backup stuff all figured out.
Flash [Wally West] is tricky . . . does he pick another speedster? An old buddy from the Titans? Since I can't think of too many other speedsters, I'll say he goes with Cyborg from the Titans.
Martian Manhunter is even more tricky . . . another alien? Another telepath? Just to mix it up, Jonn picks Firestorm as his substitute . . . extremely powerful and in need of some mentoring.
Aquaman gets a lot of flack, sure, but he's a pro. He might pick another longtime B-lister like Green Arrow, Hawkman, Zatanna, or the Atom, but I like the notion that he'd go with his former sidekick Tempest, formerly known as Aqualad.