He wants the 9 Tails infinite chakra under his control so he can sit anywhere in the world and do his bidding.
Who knows if there will be stronger people than Naruto in the future and he basically wiped out 5 of his people by himself, Naruto that is. This is on top of Nagato bleeding from the mouth from using too much of his own chakra. With the 9 Tails power he won't run out of chakra.