What kind of basic plots would you prefer in a weight gain RPG? Where would you place the main character in the story?
Some examples:
*Gods and goddesses bless the land for a mysterious reason, causing folk and creature alike to fatten up
*Tribe goes to stuff nearby locals for ancient ritual
*Fetish magician fattens up country to accrue power and enjoy the fat people
I generally like a fantasy setting, though a modern/sci-fi thing could be neat. Whichever it is though, I think it’s cool when RPGs have some neat world building to make everything feel like it’s part of a larger story, though perhaps ironically I like when the story is more contained to a smaller location with a smaller threat. Maybe have the game itself centered around a small town and how it deals with a larger crisis or even something more localized just threatening the town itself. 
Off-topic, but I find it funny how you mention a small town with a local threat and my profile is one of the biggest local threats in a story. Now back to topic.
I’ve always liked RPGs that have an affect on a moderately large space like a country or a few towns. The more I think about it, the more I feel it’d be interesting to have a map with a few towns that each have their own separate stories, or a main overarching plot and each town being effected in its own way.
Brain storming 
Well i don’t know how i would like it to go. you said alot of nice stuff!
But the lenght of the aventure will greatly influence what the story should be about. For something small and fast (like the fattening) a what if scenario can be used. Once you reach a certain lenght, you need to have a proper story, including one or more goals following each others… man i want to start so maany things because of the game jam XD
In WG related RPG’s I definitively prefer stories that are centered around one or two locations, it makes it easier to create a more personal narrative about the people that live there. The nature of weight gain already makes it so you want to see the same people multiple times, as you’re invested in making them bigger.
This works best for slower weight gain, but I could see a story that travels all over the world making people fatter as you go, but then you’d have to have 1 or maybe 2 scenes max with each character where they gain a large amount of weight at once.
I personally think that kind of story detracts from the intimacy with each character, but some people might prefer a large amount of characters getting fat fast, rather than learning to know a few fleshed out characters gain a lot of weight over a longer time.
What kind of plot it is doesn’t matter as much to me, it’s always about the characters!
Probably something already in the works- I haven’t really perused most of the games on this site- but I’d like to see a spin on the “evil sorcerer fattens up the country” wherein instead of being the hero, the player character is the evil fattener starting out small and working their way up to ruling the world or whatever. Elements of comeuppance/punishment are a big draw for me in this fetish and I think this kind of story could better facilitate that than the more typical “hero deals with multiple encounters contrived to fatten them” style.
Somewhat related, I’d like to see more games that aren’t plot reliant. Something simulationist like The Sims or Dwarf Fortress can provide an emergent narrative without constraining the player to specific actions or characters.
A game where you are the fattening menace sounds neat. Your actions could even affect the plot as you go about expanding everyone’s waistlines. Are you benevolent and supplying entire towns with surplus winter supplies times 100 or simply a deranged maniac causing chaos by forcing/stuffing/cursing people into immobile blobs.
Come to think of it, the game Lilith’s Throne more or less lets you play as a rogue magic user manipulating random locals. Granted, the game is primarily focused on transformation, but the player does have the option to alter a character’s muscle tone and body size.