Idea I had for Stardew Valley while talking to Mom:
The player either being a distant cousin, descendent, or relative of the Farmer that inherited the farm from Grandad.
Maybe the Farmer gave up after not being able to reach 100%, or maybe they just decided to move on.
The house is fully upgraded but stands derelict, rooms crumbling and in need of repair.
The farmstead is still tilled, crops persisting in random places, even machines in some spots, maybe a shed with some functional kegs and casks still.
All the upgrades around town that the farmer paid to have put in are still there-shortcuts, upgraded minecarts, the bus is still running. There's a proper school now, and the kids are all grown up, doing their own things around town.
Lewis is gone, having at some point run off with his money and cut his losses with the town. A depressed Marnie was left behind with her Ranch, animals that used to belong to the farmer now wandering her paddocks and making themselves as at home as one can be away from the farm many of them grew up on.
Sebastian, too, moved away, he's long gone, but maybe some of the video games you can play throughout the game were made by him.
In the process of him moving out, it sparked a fight with his mother and her husband, who himself ended up leaving after he decided he had studied everything he needed to in the valley, and was sick of being the singular point of contention in the house. The player only ever meets Demetrius in passing when he visits Maru for the holidays, his welcome always razor thin.
All the adults that were around when the Farmer used to be there are older, tired. There's more people, newer folks that moved in as the city expanded due to it's fane as such a beautiful valley beach town.
Vincent helps Sam and a few curators out around the museum, helping with many animal inspired displays.
And you, you're the one that the Farmer chose among their family to leave the keys to the farm, and all the objects that helped them around the Valley. They told you ti ask the Wizard whenever you need help, as well as Robin, the carpenter up the mountain.
In befriending her and her daughter(who stayed behind in the aftermath of her brother and father leaving), you gain the ability to pay for lessons in carpentry and robotics, allowing you to slowly fix up the house, the barns, the coops, the sheds and other buildings around the farm. Mother and daughter are now a perfect duo, their expertise making them the go-to to fix anything around the valley, or on the island, that breaks.
Your job is to bring the farm back to life, even in the process perhaps drawing back one or two of the other people who left the valley over time. Explore the valley, explore new areas that opened up with the exploration efforts of the Farmer before you, explore the island, and new islands.
Work with Willy, now decrepit and unable to sail his old fishing boat, to plot new areas of the sea where new fish have arisen. Fix up the boat, upgrade it into a strong sailing trawler that can weather the seas as you explore beyond the valley's shores. Find islands that were long forgotten about, meet sailors and islanders of far flung islands as you upgrade the ship to sail further and faster.
The things you can do are greatly expanded, with your predecessor having already done so much. You're free to work the farm, explore...
Carpentry and Technology/Robotics are of course new skill trees in addition to your other five skillsets of fishing, foraging, ranching, combat and farming.
Once you reach a high enough level in carpentry and technology, you can even expand your repaired greenhouse, allowing for more year round plants to be grown.
Machines to clean up driftwood on islands can eventually be made, clearing littered beaches. More island farms are to be found, allowing the freedom to have different farms for different purposes. You want an island farm just for beehouses? Once just for this crop or that? Alright.
Machines that can be used to expand your plottable land on certain farms can also eventually be made, allowing you to slowly turn untillable soil into farmable land over the course of a season.
Maybe you even have a Magic skill tree, studying under the wizard to use magic to help with your efforts. Teleportation becomes a new favorite ability, as opposed to having teleportation obelisks taking up valuable space...
All the while, you're piecing together bits of the Farmer before you's story. Why did they leave? Where did they go?
New romanceables abound thanks ti the expanded travel terrain. Befriend and then romance your favorite potential partner.
This entire thing could even be a sequel to Stardew Valley
Perhaps, even, you can, in the beginning, choose from one of your Stardew Valley saves and let the game choose how decrepit the farm is in the beginning.
All the additional exploration options are surprises, as it seems at first that you are just fixing the farm up, planning to leave it to a townsperson or give it to a family member.
But, something makes you want to stay, and make it your own.
