It would be cool if Waters appeared as a guest on the album, or had any credits at all. I mean, I know they have had big fights over the years, and they might still not be the best of friends, and they seem to have giant egos. But, it's one thing to split up in their 30's-40's over creative differences and where to take a band, and to just swallow their pride/stubbornness in their 60's/70's and just make something together. I doubt this new album means anything more than a final send off for Pink Floyd, and the chances of a new PF touring again are slim at best, so it would kinda be nice if Waters did appear on the album. I think the classic Pink Floyd lineup with both Gilmour/Waters is what most people really love.
I think egos will prevent that from happening though.