Except the problem with that scenario is that Nomura has stated that Sora is the main character for KINGDOM HEARTS until the series' end. Though I would love to see Max at some point, I wouldn't want it to come at the expense of Nomura being accused of dishonesty.
That said, I wouldn't mind something akin to a soft reboot; though I imagine that wouldn't go over well with long time fans either. Didn't work out all that well for Sakura Wars...
Poor monopolists can't win...
Decades from now, in God of War and Assassin's Creed Revelation style, we will be playing as an old, grizzly Master Sora on his quest to find out why the heartless have returned after 100 years of peace to discover that his long time wife and loving companion, Kairi, seems to be at the heart of the newfound chaos. He will set out on a quest to discover what secret Riku left behind when his heart was taken to learn that he had discovered Kairi's connection to the heartless. We find out that for the past few years the Kairi that he knew and loved was actually possessed, he doesn't know exactly for how long, she was actually Xehanort. Xehanort, a name Sora hadn't heard in 80 years ever since he defeated him. Max Goof, the new King's trusted Paladin, and Huey Duck, the last remaining member of the Quack Pack and the new King's Alchemist, join the old Sora on his quest to defeat his possessed wife and free her from the clutches of his worst enemy. To make matters worse, Sora knows he is on borrowed time and is trying to not worry his only child, Mickey Riku, and keep him from getting into the life. Meet familiar faces of those who have survived time's cruel spell, such as Roxas, who has hidden himself out on Dagobah, Namine, a witch learning new magic from Madam Mim, and Axel's son, who just earned his certification to join the Keyblade alliance and patrolling the worlds for danger. New spells, new allies, new worlds, and best of all mastery over all of Sora's combat abilities from the starts. Yes folks, this new installment promises tougher battles, more strategic approaches, and once again will test our hero's sunny disposition.