But which Assassin?

What if... Ezio Auditore joined the Templars?

Perhaps he had been manipulated into believing Lorenzo Medici, not the Borgia, ordered his family's deaths, and so joined the Templars in their conspiracy against Florence?

Or, upon hearing of Cesare Borgia's plan to seize control of the Templar Order, the Apple of Eden, and rule Italia, with the obscenely beautiful Lucrezia as his Queen, the Assassin was overcome by lust and jealousy.

Or maybe, after a long life in the Brotherhood, Prince Ahmet of the Ottoman Empire's appeals had found purchase, resulting in the older Assassin changing allegiances, and enjoying the decadence of the Ottoman court?