How about something like this.
The player has to farm up wisps for the world tree. And the play can do this by different means. Such as moving a wisp out of a maze to safety, killing enough of X creature, defend a conjurer while he conjures up some wisps, ect.
As for soul harvest, you could have the player start out as a skeleton, and his job is to raid a town, and harvest souls, and after so many souls the skeleton becomes more powerful and transforms into a ghoul, and so on.
Another idea would be to take it straight forward. The player has a farmer, and he needs to build up a farm. So he needs to get seeds, some land, a plow, water. You could take it further, and have the goal to store enough food for the winter, and have different types of crops, and have a random amount needed for each type of crop before winter comes.