What if someone took the idea for 'Myths and Legends' and really went wild with it? I'm aware that's vague at best, so let me expand on that a little.
The characters would not be reincarnations of legendary figures from Golarion, they would be those actual individuals. Of course, they would not be from the canon Golarion timeline but, rather, would be akin to drifting phantasms from alternate Golarions that never were. That would let players make alterations as minor or as major as they want without requiring too much messing around with the canon timeline and setting.
While we're on the topic of timelines though, that would be a big part of the campaign and would provide the plothook that drives the narrative. To keep things as simple and spoiler-free as possible, something has caused time itself to begin collapsing and large swathes of Golarion's landscape have been overwritten by locales from alternate timelines that are vastly different than those found in 'Golarion Prime'.
Feel free to offer whatever constructive criticism you have since this is just an idea I'm throwing out there. I believe that it's got enough potential where someone could really run with it.