https://youtu.be/tdRzphj_-cE

*Thanks for the comment IrishGoth for the feedback. Just for you guys, I will make an alternate ending*

"YOU LITTLE TRAITOR! You could've had it all! Power! Fame! Fortune! Immortality!" Bill screeched as vermillion blood gushed out of the stake wound, the head vampire turning to dust.
Mabel and Dipper watched in horror as Bill disintegrated, the stake falling into the ash pile with a "clunk."

"No! No, no, NO!" Dipper yelled out, kneeling by what remained of Bill Cipher.
Dipper's hands trailed in the ashes, scooping it up in his trembling fists, harsh sobs racking his body.

"I did it... I did it! Dipper, we're saved! You're human again!" Mabel cheered and she danced freely around the room.
Dipper didn't move, not even reacting when his skin turned a natural peach colour rather than the ghostly white hue he had before.
"D-Dipper?" Mabel asked tentatively, approaching her brother.
"You... You killed him" he uttered darkly.
"You killed the one man I loved. Mabel.... You MURDERED him!" Dipper growled at his sister, a frightening look of anger plaguing his face.

Mabel gulped.
"But Dipper.. I-I had to. It was the only way" she quavered, stuffing her hands in her pockets so Dipper wouldn't see her shaking in fear.
Her brother looked at her in disgust.
"How could you? I had a chance to kill Gideon and turn him when I was a vampire, but I held back! You know why, sis? Because I respected your love for him, your need for him! Now I have nobody..." he hissed.
"I HAVE NOBODY NOW!" he wailed as he began sobbing in anger.
"Dipper.... Am I nobody? Is Ford nobody?" Mabel asked.
She stood her ground as Dipper scowled, placing the ashes back down.
"From now, I have no sister..." he declared, his eyes on the ground.
"I don't want to be related to a murderer. Get....out....of my sight..." he ordered.
"D-dipdop?"
"GET OUT! YOU'RE NO SISTER OF MINE!" he yelled, standing up and balling his hands into fists.
Mabel sobbed hard, running out of the castle as quick as she could to escape the wrath of her 'brother', not caring that the others were celebrating their freedom, not noticing Gideon and Ford were following close behind her.
Dipper carefully gathered up Bill's ashes, placing them in an empty clay vase on the shelf, the initials "B.C" written in black pen.
He lay face down on the bed and buried his face in his arms.

Nobody heard from Dipper Pines.
Some say he ran out of town.
Others believed he still lingered in the castle all that time.
After the twins, Silas and Luna Cipher, were born, Dipper began to see things.
A lock of blonde hair on the pillow.
Those honey coloured eyes in the corner of his vision.
Some say he was going to go insane shortly.
That is until the magic of Gravity Falls started to mess with the ashes, so Bill would come back instead of resting in peace.

One dark night, when the stars were bright, the Big Dipper and triangle of stars were close to each other in the sky.
In the meantime Dipper slept restlessly, caring for his babies whenever he got up from another horrible nightmare, just to get a small glimmer of hope to keep him sane.
He felt someone's cool breath on his neck, invisible hands caressing his body.
"Bill.." he murmured sleepily.
"Come back, Bill."
His dreamy dialogue was answered by a subtle laugh, making him open his eyes.
A shadow was at the foot of his bed and was walking slowly to Bill's side of the king-sized bed.
"Did you miss me, pine tree?" a low, cold voice said in the quiet atmosphere.
"Yes...but...how? I saw you get staked" Dipper said in disbelief.
"So? They can't keep me away, Dipper" the same voice answered, a chilly hand resting on Dipper's own hand.
A pair of lips met Dipper's, enough to convince that Bill Cipher was here to stay.

(Here ya go! An alternate ending. Sorry if it's a little all over the place. I was half asleep when I wrote this)