After playing a couple of games I can't help but get the feeling the world as it is is very empty, I mean, this is fine with a game like Civ or similar, but here the lore gave us an excellent opportunity to populate it with all sorts of interesting stuff...
About a century ago all civilizations crumbled and we are left with, what, nothing? Not really, all the monsters, brigands and what not took advantage of that and literally populated everything, claiming the land, resources, ancient temples, ruins and the like...
So does all this affect our little sovereign, well, in order to bring civilization back, he has to fight wilds back, either with swords, magic or perhaps diplomacy...
So at the beginning only his inmediate area and the very first patch of fertile land would be free, all the rest would be occupied by lairs of brigands, monsters, ghosts, etc...
In this system there could be several types of lairs:
Resources lair: A gold mine claimed by brigands, etc etc... the player has to defeat them in order to be able to construct on it... they could ocasionally spam monsters as well...
Land lairs, same as before but more powerful version, they would be there to prevent expansion unless removed... Inicially though, they would be dormant, meaning that the monsters, after having had fun for 100 years are now happily spending their time in their caves counting their treasures...
Dormant means they would just spawn a weak roamer with a specific area of influence... no cities could be placed within a short range of them (after all, who would go live next to a troll lair?)
These lairs would become active though, when a city is within a certain range (bigger than the previous one)... This could simulate the monsters becoming aware of the new threat and reacting to it by spawming a much more dangerouos creature that would try to actively destroy the city in question, having done that they would return to a dormant state...
The player would have to deal with them by, well, destroying them outright and thus pacifying the area... Now adventure tech could come into play, by having certain types of lairs require an increasingly amount of tech requirements before the player can venture forth and destroy them... this could simulate things from basic dungeon exploring to specific research to break the wards of an ancient temple or ruin... (this could also be aplied to resource lairs)...
Another option in dealing with them could be a diplomacy tree (I guess this would only apply to empires for the moment) in which after having researched a certain amount the player could have the option of making a pact with the creatures (maybe through a quest, payment upfront or over time, etc) and pacify the area as well as gain new recluitment options... This could be a creature or a new type of soldier you could outfit, within restrictions of course... In the case of resource lairs the player would have to choose between getting the resource by destroying the lair or getting new recluitment options...
Last but not the last these lairs should have an incremental amount of goodies, ranging from gold, artifacts, spells, etc...
Please feel free to say what you like or not about this idea, etc. I think it could add nice amount of flavor to the early game (and prevent city spamming in the process)...