You know that show where there are all those different families all trying get their ass in the big throne? Think of that show. In their plots, they kill (or have killed) other people. So, you have a core group of people who are motivated by their greed for power. Want your world to breathe like that? Try this:
In your world, make up three of these people. It doesn’t have to three people. It should probably be three groups of people. Could be races of people, nations of people, families, whatever. OK, now determine what these folks have done to gain their current level of power (or land or money or whatever). If you work through that, you will likely come up with things they have done to each other as well as things they have done that have affected “innocent” parties. Now, figure out the affected innocent parties - nothing big; sketches work here. Now figure out what these parties will do in order to get revenge on the ones who wronged them. You’ve just written a huge and interactive history, or if you want sketched out the future action for your campaign.
What are you doing here? You’re role-playing. “If I were King Fred, and Queen Thelma from next door married my uncle in an effort to say that she deserved my throne by some rite of succession, I’d have her daughter threatened and encourage the barbarians on her eastern border to attack her prized vineyards.” “If I were the barbarians and we took over the vineyards, we’d drink all the wine, rape all the women, and carry off all the livestock, including horses and oxen.” “If I were Queen Thelma and I lost my vineyards and whole bunch of cattle, I’d have to raid Prince Mark’s lands to capture his storehouses full of food or risk my subjects starving.” “If I were Prince Mark and my subjects are all magic users, we’d curse Queen Thelma with warts, but since we can’t beat her army, we’d have to form an alliance with King Fred who still has food.”
You designed the sketch of the “people”. As you make decisions for them, you are continuing to develop them and understand them better. King Fred may have started out as one of the King Louises of France in the early 1700s - pompous and detached from his people. But as you make decisions for him, you decide if he is a Machiavellian schemer or a royal bumbler (or somewhere in between). You originally wrote that Prince Mark’s people used a lot of magic, but now you have to figure out exactly what they do with it. Can they go to war against armored knights or are they mainly scholars who are trying to figure out how the world spins on its axis? And what will they do with that knowledge?
I really think it is all about thinking through that next step. Just think through: if this happens, then that would be the natural response. It works with your world’s politics as shown above, as well as the economics, and a whole bunch of other things. It also works in reverse. You may have written a history that said this tribe moved into this land at this time. Why? Ask the next question, again and again. Why did they move in? Did they want more land? Did they get chased out? And then what were the consequences of them moving in? Did they enslave the population? Did they chase them out and they moved somewhere else? If no one was in the fertile lands, why not? Was there a war a couple decades ago and no one has taken over yet? That might need a why not question too. Ask yourself why, and you’ll really start to build.
If you can honestly (or even semi-honestly) make decisions from a role-playing perspective of “what would this character do in this circumstance”, you will continue to define the characters and flesh out your world. This really is accretive - That means the more you do, the easy it gets because it keeps adding things. After a while, you will know exactly what Prince Mark and his court wizards would do, because you’ve made a bunch of decisions for them and you know what trials they are facing on their other borders.
I hope this helps you start developing your worlds, your campaigns, and your games in general. Use the feedback you get from your players, too. They’re reactions to certain characters and ideas in your game world can help spur you in new directions, even if they are directions you did not think of moving in the first draft. Hey - It’s not retcon if they didn’t figure it out the first time around!