If it happened, Minato would definitely got to help Naruto and Hashirama would definitely go to fight Madara. I'd expect Tobirama to follow his brother, especially since he has no real connection to either Naruto or Minato. It's hard to say where Hiruzen would go, but I would guess he'd go with Tobirama and Hashirama just because it would allow me Minato and Naruto to be together without any other Kages there. I could easily see him going to help Naruto out as well though, and seeing Naruto, Kakashi, and Guy again.