I did a couple of playthroughs with an Aserai merchant character who basically does not interfere into kingdom politics at all, just making money and grinding Trade until I can buy fiefs. Then I buy them from the other kingdoms, one by one, until I take over the entire map. Obviously, the more you have, the faster you can afford the next one, but there's also some room for strategy here - fiefs that have just been besieged, or better yet looted, or generally frontline ones with villages that are constantly raided, can be relatively cheap. And, since I'm not involved in any wars, they recover pretty fast.
The amusing part about it is that you can be a single clan - not a kingdom, even - and yet fully control the map. Not only that, but AI kingdoms don't declare war on you if you're not a kingdom yourself, so you don't ever have to fight one; only skeleton garrisons are necessary to boost Security, and all the money this saves can be invested into the fiefs and caravans (which are also a lot more profitable when they're not molested by enemy lords). Similarly, all the XP clan members earn can be invested into Trade, Steward, Charm, Smithing etc, without even bothering with any combat-related attributes and skills, since the most dangerous enemies you'll be facing are bandits and deserters.
In the other playthrough, I only bought out the Aserai fiefs, and declared my own kingdom in its place. Then got all the Aserai clans except for the ruling one to join mine, using their own former fiefs as enticement. So, effectively taking over the Aserai kingdom from within as an oligarch of sorts. Of course, at that point, other kingdoms start declaring war at you etc - but it all takes long enough that you have kids all grown up by then. And for those kids, you can dump all their XP into combat- and army-related skills, and they also get the best weapons and armor money can buy from the get go, and crafted uber-items where applicable. Thus, your kingdom eventually gets a new ruler that can effectively expand it by the sword, and a bunch of martial nobles for that ruler to form the core of their army. From there it's straight-up conquest.