Here I propose two modes that could entertain players in singleplayer.
Both are in medieval setting.
1.) After world generation, when landscape and vegetation is generated there should be made an aditional generation of civilizations. Towns, castles, villages, farms, bandit camps are generated and populated with NPC characters. Each settlement and all NPCs living in belongs to some faction. There should be traders, kings, nobles, soldires, peasants. Each faction has attitude towards player that can change depending on his actions towards faction members. Factions can be hostile towards each other and wage wars, siege each other settlements. Hostile factions can attack players base.
Player can trade with NPC merchants for gold. Gold can only be received from NPCs by looting them or making their quests. Quests should be generated and be something like: kill those bandits/wolves/monster, bring me something, deliver my letter to some NPC and so on. Also player can try to rob castle/town treasury or find a burried treasure to gain gold. For his gold player can buy uncraftable items, crafting blueprints, skills, that rise his health, strength, and common resources, if player doesn't want to harvest them.
Generally speaking there should be a generated world of NPCs that lives its own life and player can interact with them.
2.) All the same as previously but the world is not static, but evolves. NPCs expand their towns, build new structures, not just repair old ones. There can be a colony of dwarfs, that developes underground city, for example.
I have a suspicion that Adventure mode (that is currently unimplemented) is going to be something like this.