The most interesting thing for me in that link is:
"I had a quick chat with Blaze in Istanbul, before his performance in Rock The Nations Istanbul. (Where he did a duet with Paul DiAnno) And in that chat he said that he had written some of the lyrics of Dream of Mirrors but Steve didn't give him a credit for that in BNW. When he saw that he had a little frustration."
I'd read that before, and ever since I've wanted to hear Blaze have a go at singing Dream of Mirrors. He would definitely sing it very differently from Bruce, but I think it would be cool to hear. And some of the lyrics really do sound like his (while some sound like Harris).
Maiden Steve seems to have a policy about not crediting anyone not currently in the band with writing a song. Paul Day (the first vocalist) claims to have written "Strange World", but he was not credited (
source).
Wikipedia says that Rob Angelo, a member in 1977, wrote "Sanctuary", but the source given is the Metal For Muthas liner notes, so I cannot verify this. The only exception to this policy is "Hooks In You", which is credited to Dickinson/Smith. If I was guessing, I would say that this is because Adrian had been in the band for 9 years, while Blaze had been in for 5, and Day and Angelo had not been in for any significant period of time.
Anyway, as to the original question, since "Blood Brothers" is out, unless it was a case where music was written, but not words yet, in 1998 (which I doubt, given Adrian's phrasing), it would have to be "Thin Line".
On the other hand, here's a thought (this is JUST a thought, nothing more). What if the fourth song isn't on
Brave New World? What if it is "When the Wild Wind Blows"? I know I'm going to get called on this, and I don't even know where to begin looking for it, but I know I've seen a video in which Harris says that he wrote that song years before and lost it, and finally found it later and reworked it to put it on
The Final Frontier. I know this is completely baseless speculation, but if Harris was remembering four songs they didn't release on
Virtual XI and Adrian told us three of them but couldn't think of the fourth... The album only has ten songs, two of which Adrian had written himself. I would think he'd be able to remember that. The conspiracy theory I'm suggesting here is that Harris said he saved four songs, but in reality, only three of them showed up on
Brave New World. Feel free to blast my little theory into smithereens.