Alright, here's the idea I was talking about earlier. Under the cut because it's quite long. It is a Gravity Falls episode idea because what else would it be. At the moment I'm just calling it "Junk Mail"
Dipper and Mabel come down to the kitchen in the morning. Stan is at the table futzing around with some mail.
STAN, putting papers in envelopes: You kids looking for something to do? Lick these envelopes.
Stan hands Mabel a large stack of hand-addressed envelopes.
MABEL: Wow, I didn't realize you had so many friends!
STAN: Friends? I don't know any of these people. You think I'm making friends with somebody named-
Stan faces the envelope he'd just finished putting mail into toward Mabel.
STAN: Joebody Smingderhoff ?
STAN: I'm sending out these flyers to the mystery shack!
Stan holds up a flyer and smiles excitedly.
DIPPER: You're sending out mail to random addresses? Does that even work?
STAN: Of course it does- Don't you remember that rush of customers we got around lunchtime yesterday?
[Cutaway to empty shack aside from the employees. Everyone's looking bored. One guy comes in. Everyone looks at him. Cut back to present scene.]
DIPPER: Still, I don't feel like putting junk in random people's mail is the best idea.
MABEL: Well I think it's the BEST idea! [Dipper looks at her funny] Think of all the joy you could spread mailing random people.
MABEL: Grunkle Stan, I think you've just opened up a whole new world for me.
STAN: See? Mabel gets it. Now put your new world outlook to use and get to licking.
Stan hands Mabel a giant stack of envelopes.
MABEL: Yeah! [runs out of room]
Dipper starts to leave empty-handed. Stan stops him with his cane.
Stan gives dipper an envelope. Dipper licks it crankily
[Gravity Falls theme song]
The episode starts with Grunkle Stan sending out spam mail of Mystery Shack adverts to random addresses. He conscripts Mabel and Dipper into licking the envelopes for him.
Mabel is inspired to start sending her own (Nice) mail to random addresses. Dipper objects to the idea, stating that people don't want mail from a stranger, and even "nice" junk mail is still junk mail.
Mabel makes and sends out a few of her nice pieces of mail to random gravity falls denizens, looking through an old Gravity Falls phone book to get addresses to do so. The next day, she and Dipper go walking around town and overhear several people talking about their letters and how happy they made them. Mabel is pleased and gloats to Dipper about how her nice "junk" mail turned out to be nice "spunk" mail. Or something.
When Dipper and Mabel get back to the shack, there is a mysterious letter addressed to her sitting on the front porch. Initially Mabel is excited to open it, but upon doing so discovers it is one of those cursed chain-mail letters. "Spread this letter to 5 other people or you will be cursed with misfortune for all eternity" yadda, yadda.
Mabel remarks it's not a very nice letter and how it really IS junk, and ignores it. Dipper looks in the journal and finds an entry about cursed mail. Apparently the author once tried to send out a survey on whether or not the townsfolk had seen some anomalous something or other and received a cursed letter in response. He documents some of the misfortunes that befell him, which Dipper reads aloud.
Mabel is adamant she'll be fine, but right after a bowling ball crashes from the ceiling and lands within an inch of her. (Stan from upstairs: "Woah! Dropped my weighted bowling ball! Hope it didn't come within an inch of hitting anyone just now!") Dipper looks at Mabel. Mable chuckles nervously.
Cut to a montage of bad things happening to Mabel as she tries to go about her day. One of these things is she gets attacked by birds in the face. Dipper and Mabel try to brainstorm how to undo the curse. Neither of them are keen on the idea of forwarding the mail, so Dipper once again checks the journal to see what the author did.
The author, taking advantage of Gravity Falls' extremely old phonebook at the time, compared the obituaries and the phonebook and only forwarded the mail to people who had died. Mabel checks the front cover of the phonebook they have only to find it's been updated. (The phone book says "Gravity Falls NEW UPDATED Phonebook!" With the subheading: No more deceased folks!") So the Author's advice is effectively useless.
Something else large and heavy and stupid falls from the ceiling right above Mabel. Dipper pushes her out of the way and the object lands in the spot where Mabel was standing with a large "CRACK" noise. They both look at the object, which has done considerable damage to the floor, then look at each other. With seemingly no other option, Mabel is forced to forward the chain mail. She draws all over and decorates her copies with stickers to try and make up for the fact that it's cursed evil mail. Dejectedly, she puts her letters in the mailbox and sets the mail flag to be outgoing.
The next day Dipper and Mabel walk around town again, passing several extremely unlucky and blatantly cursed citizens. Mabel can't stomach what she's caused, and is determined to make things right. She and Dipper decide to track down the sender of the cursed chain mail by following the return address on the letter that was sent to her.
They arrive at the Sender's house, which seems dark and abandoned. After entering they come across a person sitting in the dark and writing a letter by candle light. It is another cursed letter. Dipper and Mabel talk to them, and the Sender reveals that they're so sick of constantly receiving random junk mail that they decided they would respond to every instance of it that they receive with a powerful curse. Apparently they have been doing this for quite some time. They also reveal that they didn't even read Mabel's letter, they just threw it in their "mail box" (A cardboard box stuffed with junk mail) and responded with the curse immediately.
Mabel apologizes for sending them mail that they didn't want and starts to feel bad about her whole nice mail scheme in general. She explains that she forwarded the original message, but feels she deserves the curse and that it should all be put upon her instead of the townsfolk or spreading further.
Dipper looks back at the mail box again and sees Mabel's note sitting on top. He picks it up and hands it to the sender encouraging to read it. (Dipper: "Aside from the whole 'cursing an exponential amount of people thing', I kinda get what you're saying. ...But this isn't junk mail. It's more of... spunk mail? Er, well, just read it and you'll see.) As the sender takes the mail, Dipper puts a reassuring hand on Mabel's shoulder and smiles at her. Mabel smiles a little.
The sender looks at the twins and then reads Mabel's letter. They end up being very heartened by its contents, and remark that they had forgotten what it felt like to receive nice mail after so long. The sender agrees to lift the curse on Mabel and the townspeople completely. They also reveal they've been sitting in the dark like this because they cursed their electric company when receiving their power bill instead of paying it.
Dipper and Mabel return home happy and have an "alls well that ends well" sort of conversation in the living room. Stan walks by checking his mail. He opens an envelope with the cursed chain mail in it, apparently sent to him because of his spam mystery shack ads he'd sent out earlier. (Perhaps the Sender was Joebody Smingderhoff?) He crumples it up and tosses it in the trash and walks out into a different room. He can be heard screaming off screen as something falls on him. Dipper and Mabel look at each other.
Stan going through the phone book looking for people to forward the curse chain to. He's making fun of all of their names as he goes along.
I'm not sure if there's room for a B plot here, but if there is, then it should be that Stan runs out of stamps before he can finish his last envelope. Every place he goes to has run out for one reason or another (perhaps Gideon has purchased them all). Eventually Stan gives up on trying to find some and just opens someone's mailbox with outgoing mail and peels the stamp off it to take with him. He leaves a dime in the mailbox, seems to walk away, but then comes back and takes the dime.
