What if at some point during the war, Ardyn let himself be caught by Lucis and kept as a political prisoner and some years later, Noct during one of his rebellious phases comes across his cell. So Ardyn starts telling him things, all the things Regis has kept him in the dark about, that no one else seems to think the crown prince should know. And Noct keeps sneaking down to see him, because Ardyn doesn't treat him like a kid, actually wants to hear Noct's opinions on things, asks him what he'd do if he were the king, etc. This could go in the direction of a really questionable relationship, or it could stay gen, up to anon!