If Naruto could ever get the Kyuubi to cooperate with him, he could make himself immune to genjutsu by having the fox kick in with a surge of nasty chakra whenever he's bound by it. Only thing is, if the sharingan can control the fox, then it will likely not work against sharingan-genjutsus. Just like how Sasuke dispelled the fox that time he looked into Naruto.