Dynamic quests.
This is the only correct way to go. I think someone made a thread about it earlier but I can't seem to find it.
Basically instead of having a random event popping up asking you to kill the dragon and then the dragon cave magically appears on the map, the dragon is already there and interacts with it's enviroment. It may harras you early game or go on a rampage in a neighbouring kingdom. Maybe there are troll tribes nearby but they get scared away by the big bad dragon and migrate to a different part of the map. Maybe there's ratpeople clan worshipping some ancient artifact nearby and they have begun to mutate under its dark power. And these troll tribes looking for a new home begin a war against these mutants. Maybe you can aid one of the tribes in war learning them spells to defeat the other side. Maybe through clever diplomacy you are able to steal away the artifact. After one of the tribes win maybe you leave them alone, but after some time you suddenly realise that they have grown immensly strong and united all the trolls under a common banner due to your magical aid and you need to war with what you thought was an insignicant tribe.
Maybe you can assimilate one of the tribes and use their superior strenght in battle, outfitting them with the best of the best using their regenerative ability to beat the other factions. Maybe they begin to revolt if the trolls feel too strong and they think they should have their own kingdom. Maybe another kingdom can try to let other races you enslaved revolt to try and destabilize your kingdom.
Then in endgame maybe you can combine the artifact you stole with clever diplomacy with other artifacts to create a super weapon or maybe to protect from the world flooding, or its somehow crucial for succes in endgame.