That was the thing--Greg never specified who it was. We all figured it had to be Teridax, since his possession of the GSR gave him insight into the destinies of individual beings. But, apparently, that would be too obvious, so he stated that it was someone else entirely who came up with it. Could it have been Velika? Could be. I don't remember if that was asked. It would make sense, since the only other person who might have that kind of insight would probably be Artakha, and I see no reason for him to want to do that. Plus, having a being who can make anyone's inner desires a reality without questioning whether it's ethical or not would be of use to someone like Velika who wants to take over the world.