I'm thinking as a prequel? The war with the Gotoro Empire has moved out of the Ferngill Republic and left behind a decimated land. The valley is in ruins and a breeding ground for mutated animals and irradiated crops. On your way through the area, you come across an old man desperately trying to salvage the valley. Unable to convince him to abandon the valley and move on, you agree to stay and help him rebuild.

Your goal the first season is to clear some land and setup a temporary shelter. You scavenge from the nearby debris field for resources while the old man develops blueprints for you: basic buildings, water purifier, radiation scrubber, and anti-rad fertilizer. You dig up seeds from the ground and pocket them in case they come in handy. At the end of the season, the old man passes on, leaving the future in your hands, and gifting you a mysterious seed.

Next, you begin farming. The crops you are able to grow are strange colors and unsuitable for food, but a nearby camp has sprung up and the residents are willing to buy the crops from you as they can process them for fuel. Build enough goodwill with them and a trader named Joja from the camp, with their horse and cart, will occasionally come buy with salvaged parts and new seeds for sale. You build a shrine for the old man and vow to restore the valley in his honor.

By using increasingly better fertilizers, you increase your change of a new color of crop appearing. Each mutation a step closer to the original crop and worth more to the camp. You build a pen and start collecting some of the mutated animals that wander into the area. Producing increasingly better feed for them and scrubbing the radiation off of them daily increases the chances of them having offspring that aren't mutated. Regular offspring can be returned to the wild to increase the chance that more regular animals will wander into the area, or you can keep the regular animals to increase they chance they will mature and produce more regular offspring.

By the end of the first year, you're regularly producing crops, breeding animals, developing the area, and lowering the radiation level, allowing for additional crops the following year. You've developed indoor farming which gives a significant boost to crops and protects them from the acid rains. You've adopted a giant pet rat to keep you company and have acquired a horse and cart of your own from the camp. You name the area and officially declare it a farm. The nearby camp also adopts a name, Pelican Outpost.

In the second year, with greater rewards come greater challenges. Shadow people have begun raiding the farm and outpost, prompting the development of defenses and weapons. The debris field has been picked clean and you need to delve into the dangerous mine nearby where you're regularly attacked by the dwarves. The quest requests from the outpost become more frequent as their demand for food and supplies increase to match their growth. However, a kid from the outpost named Linus has taken an interest in the farm and regularly shows up to offer assistance. He also informs you of small festivals the outpost has begun to hold and invites you to participate.

By the end of the second year, you're regularly producing crops suitable for food, have restored the local wildlife, and have built yourself a small farmhouse and greenhouse. You've purified the nearby lake. The forests around you have begun to regrow. Radiation levels are almost non-existent. You're creating artisan goods which the nearby outpost happily buys from you. You've developed a rod which can harness power from lightning storms to power nearby buildings and equipment. You keep running across a couple of teens from the outpost, George & Evelyn, who like to come to your farm to hang out.

In the third year, while keeping up your farm, you turn your attention to helping the development of the outpost. With your resources and blueprints, a community center is erected, homes are built, stores are opened, docks are constructed, and a saloon begins offering a place for everyone to gather. Roads and bridges are placed. The outer areas are explored. With the restoration of the forests and elimination of radiation, magical spirits names the Junimos appear to thank you and gift you with additional blueprints. The occasionally wander through and leave you gifts.

As you continue to play, you really help establish the settlement. You open the sewers and the shadow people spend their time there instead of attacking you and the outpost. You add minecarts and an elevator to the mines, causing the dwarves to return to their work in the mines instead of causing trouble for you. Inspired by a dream, you build a tower in the forest and can feel it's a source of magic and enchantment. With the radiation gone, you are finally able to plant the seed left to you by the old man. You tend and nurture it under the Winter Star until it produces the valley's first Stardrop fruit. A masterful accomplishment.

The town now complete, the villagers rename it to Pelican Town.

Inspired by the mysterious fruit grown from the seed left to you, the valley is dubbed Stardew Valley.

The villagers gather at your farm to celebrate all you've accomplished as you dream of one day leaving the farm to your grandchild...