I have heard two versions of where the main lyrics for this song - 'I want it all, and I want it now' came.
According to Brian, it was one of Anita Dobson's favourite phrases, and it inspired him to write this song. link (around the 3:15 mark)
According to Norman Sheffield, the phrase began with Freddie. Here is the quote from Life on Two Legs:
"Then came a phrase he would make famous around the world in years to come, although no one would have known where it was born.
Freddie stamped his feet and raised his voice: ‘No, I am not prepared to wait any longer. I want it all. I want it now."
Does anybody know the real origin of the lyrics?
It was a phrase Freddie 'may' have said, and a phrase that 'may' have typified his attempt to break into the industry but Freddie didn't write the song. By 1989, Freddie hadn't worked or spent much, if any time with Norman for 14 years.
Brian is credited with the song but the line 'I want it all' came from one of Freddie's songs ' The Fallen Priest' if memory serves
There is pattern to Brian's songs in the late 80s if you look closely;
Who wants to live forever is a line from Flash Gordon
The phrase 'rushing headlong' was first used in Breakthrough
Perhaps these lines were used as stepping stones to new songs or maybe it was just subliminal...
'I want it all' is common enough in the English language to have probably been uttered scores of times by all four band members, their close acquaintances, their teachers, classmates, etc., for years and years.
Norman Sheffield (RIP) was just adding colour to his book by going for the extremely long shot of trying to connect himself to a Queen hit from the late 80's.
There's nothing to suggest 'Fallen Priest' (the lyrics of which weren't written by Freddie or Mike, but by Tim Rice) predated 'I Want It All.' It was released first, yes, but it doesn't mean it was written first. Sessions for 'Barcelona', 'Back to the Light' and 'The Miracle' overlapped often, and possibly preliminary work for 'Mad, Bad and Dangerous to Know' as well.
When it comes to Brian saying MFK was on 'Queen II', we can prove him wrong because there's millions of copies of both albums out there and as such it can be confirmed the song is on 'Queen (I)' instead. But when it comes to Brian saying he got the inspiration for IWIA from Anita, other than time-travelling or mind-reading, the best we can do is just take his word for it, as he's the only one who knows how the title came to his head.
Well, so according to Brian it was one of Anita Dobson's favourite phrases, and it inspired him to write this song. That should be the correct answer.
Because Sheffield has nothing to do with 80's Queen, according to Sebastian.