Rather than a PP or ED, I think that I would treat it as something akin to a "Boon" (DMG2). Give it a monetary value, as a reward. Make it something with both mundane and role playing value. That's the value that the player can draw, from his kingdom, in mundane materials, lackeys, etc..
If the player works to maintain it, rather than just soaking it, then he gets the next step of the "Boon" when it becomes appropriate. If he doesn't, then his evil half brother seizes the throne, while he's off on an adventure. At that point he's a "Deposed Ruler" or "Exiled King", and gets to role play that angle![]()
Interesting idea, thanks! Some obvious benefits are taxes, levied soldiery, and maybe the Legendary Sovereign +4 bonus to CHA-skill checks within the domain borders. I'd like something a bit more personal too.