How about a campaign with limited scope (area wise) but where everything has a consequence.
So the people you kill/help in the previous mission make an impact in later missions.
Say you save a blacksmith's child, then later the blacksmith provides cheaper stuff or equips a bunch of soldiers to help the town.
Or you kill a load of creep wolves which means rabbit population goes unchallenged and they eat the farms makes a farmer lose his land to become a bandit.
Clearing an area could cause another creep group to move into the area, failing to deal with a spider nest early could lead to infestation in later levels.
A creepy traveller in the first map could become a necromancer in a graveyard in the second map, could become a major threat in the third map then a lich in the final map.
You could have a murder mystery taking place over the course of several maps.
A bandit camp left unchecked could lead to a raid in the next map, you could also give players several ways to deal with bandits - kill them, hire them, get them jobs as soldiers or farmers.
If you've played the game Deus Ex in that there was a family - the Renton family who you could follow over the course of the game with your actions influencing what happened. Sometimes stuff you did with the best intentions ended up badly. You could have a similar family (or families) in the villages in your maps, telling small stories which the player can interact with.
In the game Dishonoured your actions through the game influence later levels, the more people you kill/chaos you cause the more sinister the world is, more guards, more zombies, worse weather etc
Then have no lose condition (aside from hero dying/losing base) but the next missions would be changed.
You can force a player to make difficult decisions, collect taxes from villagers (make them poor and angry), pay out of your own pocket or let someone else collect the taxes (maybe they'll mistreat the villagers or use the money they gather for something nefarious). You could use a powerful evil artifact but is it worth the sacrifices it forces you to make?
For sake of narrative/story you'd probably want major events which the player interacts with, a civil war, a necromancer becoming a major threat, a famine, a disease, a dragon waking up, a werewolf spreading it's disease, a vampire having it's offspring.
You could handle it by having reputation with various different factions (like helping out a village) along with flags/variables for individual events (like "bHasSavedBlacksmithChild" or "iWolvesSlain" or "iNorthVillagePopulation" or "iWerewolfPopulation").
The map could take place in a forested valley with a few villages and a fortress over the course of a year. 4 maps, starting in spring, summer, autumn, winter.
