It practically writes itself. John and Aeryn are out for a little joyride in the module and whoops, wormhole, which takes them to 1940s Earth (because John hasn’t quite figured out how to navigate them 100% yet). John was a huge Captain America fan growing up and maybe he had regaled Aeryn with tales of a woman named Peggy Carter, that he had the pleasure of hearing about from his dad and actually got to meet once, and well, Aeryn definitely needs to meet this hooman that sounds like she would have made an excellent Peacekeeper. Except in this reality, Peggy doesn’t live long enough to form S.H.I.E.L.D. and so they decided it’s ok to bring her back with them to the ship?
So Peggy gets translator microbes, which extend the lifespan of the individuals who get them, and somehow they end up near Earth when the Chitauri invasion is happening and Peggy and Aeryn take the prowler down and get involved in the fight, where of course they see Steve and yay happy reunion!