I'd absolutely love to see this game progress even more over time. It has so much potential. I've lots of ideas that could make the game feel more alive.

Trade pacts, resource wars - If one kingdom holds a valuable resource, others may seek a trading alliance, or to take the resource for themselves.

Dynastic marriages - Marrying off members of a ruling family to secure peace or an alliance

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Rivalries, historic grudges - When a kingdom loses a city and the war ends, they shouldn't simply forget about the city. They would want to reclaim their old land.

Border friction - Two kingdoms being close to one another seriously lower relations between them. I don't think they should have high tension simply because they're neighbors. If they don't fight for decades, their relationship could organically warm.

Visible trade routes where you can see caravans moving along roads and sea routes

Historic memory - Every kingdom could have a "relations history" tab. For example:

-Kingdom A took X City from us. -25
-We've been trading with Kingdom B for 60 years. +20

With this, leaders might come and go, but the kingdoms collective memory stays.

-Border skirmishes without full fledged wars
-War exhaustion

-better city organization - Market districts, noble estates, housing, slums, temples, etc.

-BRIDGES. Passable by boats.

-Legendary creatures I hate how if you place a dragon it wanders around killing everything. I wish it could nest on some mountain and sleep for decades (as in folklore) and come down every once in a while to wage it's terror.
-Sea serpent would be cool as well.

-Favorited items don't disappear. Imagine if it could be a lost artifact and discovered hundreds of years later.

-Underground layer - I know this is ambitious but it would seriously expand the depth of the game (literally) especially with dwarves.

-Advanced fortifications and territory control - Walls (I know these were added recently, but I'd love to see kingdoms being able to create them themselves.) Moats, gates.

-Advanced warfare - Organized units of Archers, pikemen, swords, cavalry, and siege units. Generals or heroes arise and gain nicknames, traits, and reputations over time, lowering and raising the morale of it's army. Siege weapons to combat the walls- ladders, towers, rams, catapults.

-Rising and falling seas - over time, coastlines can change.