This was discussed in an interview with Today.com's Jen Brown. Apparently Arthur Weasley was the one who was originally slated to die but got a reprieve, largely due to the author's desire to keep alive the one genuinely "good" father figure in the novels. Unfortunately Remus Lupin and Nymphadora Tonks then had to die in his place, to allow the creation of another orphan and thematically bring the story back around full circle.
“If there's one character I couldn't bear to part with, it's Arthur Weasley,” Rowling admitted for the first time publicly in an interview with TODAY’s Meredith Vieira. Hence, in “Phoenix,” Mr. Weasley survives a snakebite … just barely.
“I think part of the reason for that is there were very few good fathers in the book,” said Rowling. “In fact, you could make a very good case for Arthur Weasley being the only good father in the whole series.”
The author admits that just as Dumbledore became attached to Harry,
she became too attached to Arthur Weasley. But there is another reason
she selected the two additional characters, who had survived in her
original vision of the story, to die at the end of “Deathly Hallows”
in Mr. Weasley’s place.
“I wanted to kill parents,” she said, quickly adding that sounded
“terrible” to say. “I wanted there to be an echo of what happened to
Harry just to show the absolute evil of what Voldemort's doing.”
The theme resonates throughout the books with the deaths of Sirius
Black and Albus Dumbledore, Harry’s flawed father figures. And that’s
why, in the Battle of Hogwarts, Remus Lupin, Harry’s only remaining
father figure, and Nymphadora Tonks die, in the process creating
another orphan in their son, Teddy.
Rowling: I wanted to kill parents
She also spoke to the issue in a 2007 interview on Dateline
JKR: When I sketched out the books, Mr. Weasley was due to die in Book 5.
MV: So what happened there? Why did he get the reprieve?
JKR: Well, I swapped him for someone else, and I don't want to say who, for the people who haven't-- read. But I-- I made a decision
as I went into writing Phoenix that I was going to reprieve Mr.
Weasley and I was going to kill someone else. And if you finish the
book, I-- I expect you probably know and someone else who is a father.
Because I-- I wanted there to be an echo of-- of Harry's loss of
parents. And you probably know who I'm talking about if you've
finished the book. But-- so there are two characters who are killed
instead in Seven. So Mr. Weasley did get attacked, as you know, in
Five. But he would have died if I'd have stuck to the original plan.
But he survived. I had to keep him alive partly-- partly because I
couldn't bear to kill him.
MV: But there were two that weren't supposed to die that did end up dying.
JKR: Yeah, yeah. I swapped them for Mr. Weasley. But they didn't then die until Seven.
MV: So as an author, then, there were certain characters you couldn't bear to part with?
JKR: If there's one character I couldn't bear to part with, it's Arthur Weasley. And I think part of the reason for that is there were
very few good fathers in the books. In fact, you could make a very
good case for Arthur Weasley being the only good father in the whole
Vieira, Meredith. "Harry Potter: The Final Chapter" Dateline (NBC) , 29 July 2007.