There’s a Good Goodbye demo floating around that has an extra verse from Mike, but it wasn’t officially released. Most demos released after Hybrid Theory are officially done through the band’s collection. Older stuff popped up in the past because the band would be sending CDs and tapes around for various reasons that eventually ended up online
Judging by how things have been going, I don’t see there being one (music-wise).
Don’t get your hopes up.
Who do you mean by ‘they’. I’m guessing you don’t mean the band since the answer is blatantly obvious.