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Itachi & Nagato
The previous Jinchuriki
Someone else (Specify)
Seeing as Madara now has six Paths to act as a substitute for himself, Naruto fighting Madara wouldn't need to be them fighting directly. Naruto could very well take out the Jinchuuriki Paths, causing Madara to flee. Or Madara sends out the Jinchuuriki Paths while he absorbs the Juubi or something. There's quite a few possibilities.
Those would be fine. But having him lose to Naruto and flee isn't the problem. It's having him flee and then there being no way to power himself up so that the "real" fight between Naruto and Madara feels dire. If we know he can defeat him already and he flees, then the next time they meet the fight will be boring and anti-climactic because he will have gotten ahold of nothing that will make him stronger than he is currently.
If he fights, loses, and returns, then he better return with a Super Ultra Mega Pein with 6 God Realms or else it's a waste of a fight.