Avengers: The (Alien) Sun's Going Down

Natasha had a feeling Bruce would run, and she had been very serious when she said she'd run with him. Unfortunately, she hadn't planned on running across the universe to a literal (and figurative) dump. Oh well, they could make this work. Various pairings. AU for AoU, CA:CW, S:H, T:R, BP, A:IW, & AMaW.