Power structures and their fault lines
Political fantasy's world-building starts with power structures: who holds power, on what basis it is held, what legitimizes it, and where the fault lines along which it might fracture run. A monarchical system might be contested along lines of succession, legitimacy, or the relationship between crown and nobility. A council system might be contested along factional, economic, or regional lines. A theocratic system might be contested between different interpretations of doctrine or between religious and secular authorities. The fault lines are where the story lives: the specific disputed successions, the specific doctrinal controversies, the specific resources that different factions control and contest. Building the fault lines before you build the institutions produces a political world that is already in motion when the story begins.