Rareware was one of my favorite developers, but the company was really a mess during the sixth console generation. It's too bad, because I would have loved to see how the older builds of Perfect Dark Zero and Kameo turned out for the original consoles they were planned on. Obviously, you can still get an idea of how they would've been through screenshots and videos, and the games did get released eventually (Kameo was decent on 360, but PDZ was a big disappointment as the reason I bought the console). I'm sure most people know about the effects demo for PDZ on the original Xbox, but I wonder will a prototype ever emerge? I can't imagine development was ever even started on the Gamecube, but I don't think a prototype of a Rare game has ever been leaked anyway, has it? Unseen64 has a fair bit of interesting content on PDZ. With Free Radical being formed, it seems like the best of the crew ended up working on the Timesplitters games, so I've always considered TS2 and Future Perfect to be the true successors to the Perfect Dark/Goldeneye throne rather than PDZ. Having said that, I still would have liked to how the Xbox version differed from the released one, if at all.
It's hard to say if a prototype ever will pop-up. In reality such a build should never see the light publicly. However there are a couple of Kameo builds for XBOX1 floating around, so I would not be amazed if the same counts for PDZ. But in the end, only time will tell. I do remember seeing some XBOX1 screenshots of PDZ, let me see if I can find them.
http://www.unseen64.net/2008/04/16/perfect-dark-zero-xbox-proto-cancelled/ The last screen they have is a little mysterious to me, because I've never seen a shot that looked quite as complete as that one for the original xbox.
