I had an idea for a map, but I can't come up with interesting quests for it, and then you just sit and think: "No, I'd better come up with some other idea for a map."

Think through the plot as thoroughly as possible and implement it in the form of tasks. Quests should not be just for the sake of having them; they should be an integral part of the map. These are the tasks that your heroes face in the course of the conflict you have invented.
Who is a friend? Who is an enemy? What is the ultimate goal? What are the characteristics of the distribution of forces (territory, mobility, size and professionalism of the army, the possibility of recruitment or defection, strongholds, key points and important resources, including human resources)? Do the heroes have a plan to gain maximum advantage? Do the enemies have a plan that could interfere? Is this a local conflict or is there a possibility of intervention from outside? Perhaps not from certain people, but from certain circumstances – a plague, a fire, a flood, a meteor fall, five times more wheat than usual was harvested, but rats arrived, blacksmiths announced Oktoberfest and went on a binge...
If the plot does not turn out to be too elaborate and branching, consider making a multiplayer game based on it, rather than a single-player game. There will be no such requirements for the plot and quests; they can still be made interesting, but you won't have to come up with a large number of elements.