Recently backed Pathfinder Kingmaker, so now I'm curious....
A kingmaker game that works? Pretty hard. I've started in several that never made it very far. I've run it for an RL group before and we managed about 1.5 books before it got meh. To some degree, it suffers from the design since it gives players limited direction. To a different degree, it suffers from the kingdom rules being a little spotty in places. There's also a bit of a disconnect between the kingdom and your characters - there's not much of a personal benefit from, say, being the king. There aren't armies in the first few books that you can raise (though I hear there are later) and shuffling around some magic item availability is... a lukewarm benefit.
At its heart, Kingmaker has a "hex crawl" feel to it since you're basically doing just that. You move to a hex, explore it, and then move on. Over a pretty big map, so there's a lot of repetition there. Later on, there are more meta and coordinated events, but I never really got there.
Alllll of that said, it does have the heart of an interesting game to it. I just think I'd heavily revise it for PBP play by removing most of the hex crawl, making the opening section more linear, and putting some more focus on the kingdom aspects once they open up (they don't immediately).
I'm... tempted to try, actually.