Honestly, I think the fourteenth member was his idea. He always said he pictured a girl being responsible for Roxas leaving (which is dumb and bleh but whatever). I don't think Kanemaki was the one who came up with the fourteenth member, I think she was just the one who came up with the details of it (as in, she's a replica, her name means this this and this, she looks like me)I almost guarantee you that Nomura heard the idea of "A secret 14th member of the Organization" and with his love of plot twists simply for surprise's sake, included it. That's one thing about Nomura's ideas that I don't care for. I'm all for shock and awe, but Nomura does random crap just in the hopes that we'll gasp at it.
Keep in mind that Nomura also came up with the idea for the just as fanfic-y X-Blade, aka the WEAPON EVEN STRONGER AND MORE IMPORTANT THAN THe KEYBLADE~~~ So, yeah.
It's a shame when he gets so focused on shocking people and doesn't focus on delivering strong character stuff because when he does it's usually pretty good.