You just can't beat the GNR from 87 - 92 (think that's when Izzy left off the top of my head.) Since then all that's been released is a cover album and two contributions to two soundtracks.
Meanwhile Scott Weiland has written vocals to appear on 7 albums and Slash has released material for 3 complete albums that I know of. I like the songs that have been "released" so far but if I was a fan since 87, I would not think that any song or album would be worth a 15 year wait.