I thought about this issue too and researched some time ago. I found half of the answer but not the whole one. So JKR told us in BBC Newsround, 7/8/2000 interview there are two plot holes.
The famous plot hole. I got halfway through my plans and realised
there was this huge gaping hole in it, there's two - it just didn't
meet and that was entirely my own fault, I should have had the good
sense to go through it very, very carefully before I started writing
but I hadn't.
So I'd written what I then thought was half the book it turns out to
have been a third of the book before I realised that this wasn't going
to work, so I had to do an enormous amount of unpicking, and in the
unpicking process I'm afraid the Weasley got [draws finger across her
First part is Ron's cousin Mafalda Weasley, it was explained in this interview. Ginny was supposed to be first girl born in the Weasley family in several generations.
You sat on the title for a long time, too.
JKR: The title thing was for a much more prosaic reason: I changed my
mind twice on what it was. The working title had got out – 'Harry
Potter and the Doomspell Tournament.' Then I changed Doomspell to
Triwizard Tournament. Then I was teetering between Goblet of Fire and
Triwizard Tournament. In the end, I preferred Goblet of Fire because
it's got that kind of “cup of destiny” feel about it, which is the
theme of the book.
Was this the hardest book you've had to write so far?
JKR: The first three books, my plan never failed me. But I should have
put that plot under a microscope. I wrote what I thought was half the
book, and “Ack!” – huge gaping hole in the middle of the plot. I
missed my deadline by two months. And the whole profile of the books
got so much higher since the third book; there was an edge of external
And what exactly was that gaping hole all about?
JKR: I had to pull a character. There you go: “the phantom character
of Harry Potter.” She was a Weasley cousin [related to Ron Weasley,
Harry's best friend]. She served the same function that Rita Skeeter
[a sleazy investigative journalist] now serves. Rita was always going
to be in the book, but I built her up, because I needed a kind of
conduit for information outside the school. Originally, this girl
fulfilled this purpose.