does toady plan to expand on that system somehow?
The answer to this question in almost any context is yes.
Right now there really isn't much point to becoming a barony, county or duchy, nor is there much point in becoming the mountainhome.
By my understanding it used to be, in much older versions of the game, that these were effectively ramping up the difficulty of the late game since nobles were more numerous and somewhat more of a burden on the fort. The king could also arrive without warning if certain conditions were met (mining adamantine I believe could trigger it even for tiny forts), and he'd suddenly make a big imposition on the fort without contributing anything in turn.
Eventually I would expect that becoming the mountainhome would open up a lot of diplomatic and warfaring options. In the upcoming release we're just now starting to see the potential of sending out squads to raid places off map, so it's a way off yet, but I could certainly see the ability to formally declare war on neighbors being something you could do in future versions if you had a landed noble or royalty in the fort.
how do you go about getting a king and baron?
I've never had the king show up, so someone else will have to chime in on that, but for a barony it's usually easy enough that it happens on its own as the fort grows. From what I recall there was just a population minimum (80?) plus some minimum amount of created and exported wealth. Usually you achieve all of the requirements pretty quickly without trying.
Unless it changed in recent versions, the liaison would announce in a meeting that you had the opportunity to accept being a barony once he arrived with the caravan after you hit the needed milestones. If you reject the opportunity, he'd leave without letting you set up any trade agreements. The process would repeat every year until you accepted it.