He was obviously a key component. However in 2016 if I look at everyone's output of material since the breakup I by far prefer what Axl, Slash & Duff have done in comparison to Izzys output. Now I'm not saying for one minute that means he couldn't compliment those guys just like before however musically I think Slash has shown some strokes of genius that if Axl had been involved in would have been even better.. I'm talking Fall to Pieces, Anastasia, Starlight, World on Fire, Bent to Fly etc
If Izzy is involved great but if not I'm still extremely hopeful. Spaghetti incident certainly still sounded like GN'R despite Izzy not being on it.
I think Izzy writes songs that are better sung by someone else. If you imagine Axl singing some of those songs it sounds pretty cool, add slash on guitar and it already becomes something else.
Its like Bob Dylan, his songs are cool and all, but very often, when other people cover him the songs become catchier, vocals are better, and the whole songs just sounds better.
If you study Izzy's songs, he's got a real craft for making, not just a song, but a rock n roll song in the classic sense. With izzy's structure compositions, I think they could do wonders in this reunion.