Uhm.... I think you just answered your own question. Revolution happens; protagonist is eventually forced into compromise with her opponents to create a constitution and an Office of the Prime Minister. Also, presumably, this creates a legislature of one form or another. Might want to look up England since that's the model you appear to be using. Or any of the other constitutional monarchies that exist in Europe, like Sweden.
The form the government takes doesn't necessarily have to be different simply because magic exists or goblins are an issue. That's not the kind of thing that affects such matters. What matters is the fact that the lower classes (mostly the merchant classes in your specific example) feel enough resentment that they'll risk death to attain their goal. And, if this is mostly the merchant class running things, this doesn't necessarily mean that rural farmers are being considered by either side all that much. Look up the Magna Carta; it conferred rights and such mostly to barons, who were nobility and it didn't do much to confer rights to the peasants. Your situation doesn't have to automatically jump from mideival hellhole to near modern day democracy. It can be a step on that path to it.

Something inspired by Terrible Writing Advice:
Beaubien makes a point in his video that the natural enemy of a Deadly Decadent Court is non-aristocratic classes figuring out ways to get representation in the nobility, such as merchants being married in more for business ventures than wealth. That actually set off my brain a bit, effectively conjuring in a Well-Intentioned Extremist Evil Chancellor who started off as The White Prince; exposure to how horrible the peasantry is treated in his world, and in his kingdom in particular, caused him to lose faith in the concept of aristocracy as a whole. His ultimate goal is to manipulate the two major houses, who appear at first to be "the good guys" and "the bad guys" (the latter of which he has blood relations to) into destroying each other's credibility-and from there provoking a revolution with his actual allies, the technically Corrupt Church (as in, their desire for more social mobility is genuine, but their means are underhanded) and a resentful growing merchant class and force the creation of a constitution that greatly limits noble power and creates an Office of the Prime Minister (naturally, he plans to be the first elected or the actual first's trusted advisor, but he wants to be actually elected).
Naturally, I want him to lose the personal battle, but win the war of ideals-the protagonist eventually comes to see where the chancellor is coming from, and she comes to realize she's been supporting a corrupt system that breeds abuse of the underclass and wasting lives on pointless vendettas. So, after he's beaten, she actually gets her forces to negotiate their surrender to the rebel army and draft a constitution, with her own caveats to counter what made him an antagonist in the first place (his own elitism and belief that, ironically, Democracy Is Bad).
So, my question is, how do you think this would manifest in a world with relatively accessible magic (which here can be defined as "a force you can tap into if you can pay the price, and/or are willing to make your entire life revolve around it"), and with a distinct presence of hostile monsters? What kind of concessions would one make for the mage guilds and their weirdness, and fighting off angry goblins (a species of sentient tree pollen-the reason they come out of the woods is that some of those trees are the females of their species)?
Edited by Leliel on Nov 10th 2018 at 8:22:11 AM