Umbridge and Ginny for Fred-because I absolutely ADORE Fred and I don't think even 2 people can compare to Fred. I would swap anyone for him except George.
Draco Malfoy for James Potter because we could see Lucius VS James instead of Draco VS Harry, and there would be another Slytherin to step up to the challenge of a missing Draco Malfoy. Maybe Crabbe...
Molly Weasley for Lily Evans because we would still have a mother figure in the books as well as Lily and James Potter.
Dudley for Sirius Black because... well, I dunno. It just kinda works in my head....
Arthur Weasley for Remus Lupin because I think I remember reading somewhere Jo was thinking of killing off either Remus or Arthur and I want back Remus.
Andromeda for Tonks because Tonks could come back with Remus and Andromeda has no real purpose other than taking care of Teddy, which Tonks and Remus could do if they were alive.
I would have traded anyone to die instead of fred because 1 i love him and 2 if he hadnt died, him and george could have traveled around the world to spread laughter and george might not have gotten married and would have changed everything