What if Mabel had made a different decision at the end of NWHS? Spoiler alert. All characters belong to Alex Hirsch, not me.
Pushing It
Dipper and Soos fought with Stan in the air.
Dipper called out, "Go! Mabel, press the red button! Shut it down!"
"No, you can't!" called Stan. "You gotta trust me!"
Mabel was crying, and the tears from her eyes floated up.
"Grunkle Stan, I don't even know if you're my Grunkle! I wanna believe you, but-"
Stan said, "Then listen to me. Remember this morning when I said I wanted to tell you guys something?"
The computer countdown reached T-minus twenty seconds. The portal flashed and Dipper, Soos, and Stan were pushed against the wall. Mabel hung on to the pedestal and prepared to push the red button.
Stan said, "I wanted to say that you're gonna hear some bad things about me, and some of them are true, but trust me. Everything I've worked for, everything I care about, it's all for this family!"
Dipper said, "Mabel, what if he's lying? This thing could destroy the universe! Listen to your head!"
"Look into my eyes, Mabel! You really think I'm a bad guy?" asked Stan.
"He's lying! Shut it down NOW!" yelled Dipper.
"Mabel, please!" called Stan.
The computer counted down, "Ten. Nine."
Mabel went for the button again, then looked up at Stan.
"Grunkle Stan..."
The computer continued its countdown. "Six. Five."
"I trust you... and..."
She pushed the button. The Portal shut off with one more huge gravitational glitch, then everything crashed down.
Dipper, Stan, and Soos fell to the ground.
"Yay Mabel!" cried Dipper. "You saved the world!"
"Nooo!" cried Stan. "All these years, all I've worked for, all for nothing. Mabel, WHY?"
"Grunkle Stan, I believe you're a good man. So I know you never read the warnings we just found with the black light," said Mabel. "You wouldn't have opened the Portal if you had."
She lowered her eyes, unable to look at the intense pain on his face.
"What warnings are you talking about?" asked Stan in a hoarse voice.
"You knew perfectly well and you didn't care," said Dipper. "It said, 'The device, if fully operational, could tear our universe apart! It must not fall into the wrong hands. If the clock ever reaches zero, our universe is doomed!'."
"I read that and took it with a grain of salt," said Stan. "My brother got a little paranoid before he vanished. I was willing to take the risk. All I wanted was to save my brother; now he's lost forever."
"Your brother was the Author, and he's missing?" asked Dipper. "You risked the whole universe for one guy. Are you insane? Or just lying again?"
"You just wanted to save your brother, and I stopped you?" said Mabel. "Now I feel really bad."
"He was your real Grunkle Stan," said Stan. "I've just been using his name. I'm actually your grandfather, Stanley Pines."
"Yeah, right," said Dipper. "Our grandfather died before we were born. About thirty... thirty years ago?"
"That's when your Grunkle Stan was lost in the Portal. I faked a car crash, and my own death, so I could take Stanford's identity and use the Portal to find him."
"It all fits," said Mabel. "That article we found. They never told us how our grandfather died."
"But grandfather or not, he's a criminal who nearly destroyed the world," said Dipper. "You did the right thing, Mabel."
"Then why do I feel so terrible?" asked Mabel.
"You feel terrible because you ruined my life," said Stan.
"I'm feeling pretty terrible myself right now," said Dipper. "I've spent half the summer being duped, cheated, and used. I've had it. I just want to go home."
"What about the Mystery Twins, and solving the rest of the mysteries here?" asked Mabel.
"Mystery Twins! What kind of detectives are we, anyway? Everything was happening right under our noses and we didn't see it. It took professionals to point out the clues," said Dipper. "I say leave the rest to the professionals."
"You can't just give up," said Mabel.
"Yes, I can. I was going out of my mind trying to work this out. I lost sleep, I almost sold my soul to get answers. And all the time, 'Stan' here knew everything, and he was laughing at us behind our backs."
"No I didn't, and no I wasn't," said Stan.
"Everything. Where all the journals were, who wrote them, what was the secret of Gravity Falls. It was all right under our noses. He knew it all. Why bother to share any of it with a couple of stupid little kids?"
"I wanted to protect you," said Stan.
"I've heard that before," said Dipper.
"What about Bill Cipher?" asked Mabel. "Don't we still have to stop him?"
"Stan was probably working with him," said Dipper. "I'll tell you one thing. That time of darkness when everything we care about will change? It's here, now."
"This isn't it," said Stan. "An Apocalypse is coming. It's going to be horrible. Getting my brother back was the only chance to stop it."
"You're lying to make us feel bad. Just quit it," said Dipper. "Soos, hold him here. I'm going upstairs to call the police."
"Dipper, no!" said Mabel. "You can't turn in our Grandpa Stan."
"You still think he loves us? What about this? He was bringing in barrels of toxic waste right through the gift shop. You and Waddles roll around on that floor and sometimes even eat off it."
"Grandpa Stan, you could have made Waddles sick," said Mabel. "I hate you after all. Get the police, Dipper."
Agents Trigger and Powers came down in the elevator with Dipper.
They looked around in astonishment at the underground facility.
"The doomsday device," said Powers. "This is what was causing gravity to go haywire?"
"Yes," said Dipper. "And it would have destroyed the world if Mabel hadn't shut it down."
They cuffed the miserable and silent Stan with his hands behind his back. Then they cuffed Soos and read him his rights.
"Soos didn't know anything!" said Mabel. "He was helping us."
"Yeah, dudes," said Soos. "Everyone will tell you I'm completely ignorant of everything. Except for video games. I know a lot about how to get frogs across the street, and punch leopards."
"You're implicated as his accomplice in several shady actions, like rigging carnival games," said Powers. "We have to take you in."
"What about us?" asked Dipper. "I guess you'll send us home?"
"It's not that simple," said Powers. "Child protective services will want to investigate how your parents could have sent you into this kind of situation."
"They didn't know," said Mabel.
"They thought he was our Grunkle Stan, who had a good reputation," said Dipper. "Not a criminal impostor."
"They should have checked up," said Trigger. "Agents have already gone to your home and found a letter from you pleading for help. 'Our Uncle's house is crazy... Send help and/or weapons'. You wrote that, didn't you?"
"Yeah," said Dipper, "But..."
"Things got scary sometimes, but we liked it here," said Mabel.
"You don't realize how neglected and abused you've been," said Powers. "Probably it was the same at home. Zero supervision. You've developed into quite a pair of delinquents."
"Delinquents? Us?" asked Dipper.
"You spent a night in jail for helping your criminal guardian make counterfeit money," said Powers. "Today you set off illegal fireworks. And ever since we've been following you there's been a trail of break-ins and vandalism everywhere you go."
"We didn't hurt anyone," said Mabel.
"The mini-golf course you broke into in the middle of the night for a grudge match was so damaged they had to shut it down. You also trashed a pizza place at the mall," said Powers.
"We were fighting monsters," said Dipper.
"You staged a puppet show and set off fireworks into the audience," said Powers. "Before that, our listening devices heard you talking crazy to yourself and throwing yourself down the stairs, young man."
"That was because of a demon," said Dipper. "You need to know about him."
"I don't want to hear more of that stuff from you. Not after you set off a zombie attack that nearly killed us," said Trigger.
Powers said,"With all the charges against you, you'll most likely do time in juvenile detention, if not adult prison. They have crazy laws in this town and everyone is tried as an adult."
"Dipper can't go to that prison. Gideon has friends there that would hurt him," said Mabel. "I saw it on TV."
"And if you're lucky enough to stay out of prison and detention, you'll probably go into separate foster homes," said Trigger.
"Separate foster homes? No! I want to stay with my bro-bro!" said Mabel.
"Not if your parents are found negligent," said Powers. "You'll have to go through the system. It'll be good for you; it'll straighten you out."
"You'll have to be handcuffed in the car on the way to protective services," said Trigger. "We don't want any repeats of that car accident stunt."
"We save the world and look what happens to us," said Dipper bitterly. "No good deed goes unpunished."
"Maybe it wasn't really a good deed at all," said Mabel. "I wish I had listened to Grampa Stan and not pressed that button. Everything is miserable."
"But Mab-"
Dipper's angry retort was cut off. The agents holding out handcuffs froze in place.
Blendin Blandin appeared in the room, camouflaged like a beach.
"The Time Baby isn't happy with this outcome either, Mabel," said Blendin. "I'm here to offer you a do-over."
"But if I don't press the button the world will end!" said Mabel.
"No it won't," said Blendin. "That danger was exaggerated. You need to bring back your great-uncle Stanford and stop Bill Cipher."
"Dipper won't like it," said Mabel. "He'll be made at both me and Grandpa Stan."
"Sometimes when you do the right thing people get mad," said Blendin.
"All right, take me back," said Mabel. "Whatever happens can't be worse than this."
"Mabel, what if he's lying? This thing could destroy the universe! Listen to your head!"
"Look into my eyes, Mabel! You really think I'm a bad guy?"
"He's lying! Shut it down NOW!"
"Mabel, please!"
"Grunkle Stan..."
Mabel let go of the podium and floated in the air.
"I trust you."