I love the Rune Factory series, which is basically what you describe: You are a farmer by day, but have a true RPG experience, with skills, levels, dungeons, storyline, etc, at night.
Now --- why would the player do this?
The player starts with farm animals, and they sometimes vanish at night.
Some of the farm animals are carnivores, and you must feed them fresh meat or they will eat your other farm animals.
The player is a werewolf, and the hunting is to keep the town safe ---- BUT s/he cannot be spotted by townspeople when in wolf form, or s/he'll be hunted down.
I like the last idea best, but it could be combined with the other ones. Perhaps the player is a werewolf and is, well, growing a horde of other horror movie monsters to take on the Big Evil. Said monsters need to eat meat or blood, too...perhaps to heal.
Also gives you a Pokemon flavor --- other party members become the farm animals, and grow through battling.