No idea. Presumably they follow some sort of pattern similar to whatever criteria they used to determine the default Shep if you start a new game. Which maybe leans towards Renegade? As for the ending, I would guess that
the Destroy ending would be the most logical. I can't see that Synthesis would be likely as if they stick to the EMS requirements they some players wouldn't qualify.
Edit: Angry Joe has put up a list of problems with the endings. Covers all the main points most people have. Extreme spoiler warning obviously.
He missed one it seems. All the others were bleeped. The title is "10 Reasons We Hate Mass Effect 3's Ending" for those that want to search for it.