Imagining your own fictional world requires imagining not just the rules for your world, but who (or what) rules your world. You are obliged to create some system of organization for your community of characters, even if it as simple as a local chieftain whose word is law. Designing any type of government means that your book will be inherently political.
Often the government plays a major part in speculative fiction works about alternate histories and dystopias. It is much easier to have an exciting plot when a there is an evil government that needs to be overthrown. Authors do not devise all this villainy on their own. Rather they look at past political menaces and there are many variations from which to choose: governments like Ancient Egypt with a pharaoh who claims to be a god, governments that resemble the Roman empire, with a decadent upper class, the bread dole for the poor, and the violent entertainment of gladiator circuses, genocidal governments, in which people were encouraged to inform on neighbors, such as the third Reich of Nazi Germany.
You may not want to use a past empire to but invent something new. However, looking back at history will help this endeavor, also. Recall the Third Reich? This term basically means the third empire. The First Reich referred to the Holy Roman Empire that existed during the medieval era and the Second Reich was the Prussia Empire, which covered more than half of current Germany and lasted until World War I. If you look as how each of these empires built on past one, and yet changed the nature or their politics, you may be able to look forward and create a logical progression that yields the Fourth Reich.
If you decide to scrap the whole idea of a political system and create a plot without one, you’ll be attempting to do what has not really been accomplished before. No government means anarchy—one does whatever one likes. A book about a post-apocalyptic world may start out at the point in which order has broken down, but readers prefer a plot, which means characters deal with a problem and for the world to remain an anarchy solves nothing.
So, what kind of government will it be? You must choose carefully because whatever your own sentiments about whatever type is depicted will become apparent. This is an inescapable consequence of having the freedom to leave reality behind in your writing.