EDIT: Forgot to add Asriel, he's here now.
So, I've come up with an idea of sorts, an Undertale AU. I've been adding onto it and I believe it's time I share the idea.
I call it Undertale 0.5, an Timeline where W.D Gaster still existed, of course with it's own head cannons. Plus, no, I don't mean Gatser existing in the past, I mean if he was actually an major part in the main game of undertale. The main plot is still the same, but with an few changes of course. Chara and Asriel switched fates, sans is an assistant of Gaster, Papyrus is an low class member of the Royal Guard, and so on.
FYI: Mettaton does not exist in this timeline, but his ghost alter ego "Hastapblook."
FYI 2: There will be an comic of this AU soon.
Frisk: Frisk is the same, but now has an voice they use to talk to others. They mostly stay silent though, especially in an Genocide run. The part they talk the most is during the True Pacifist Ending.
Chara: Chara has adopted Asriel's fate of becoming an flower, after the shards of their collapsed SOUL fell onto an out of place Sunflower. They now take the name "Sunny The Sunflower," much more cunning and smart than the Classic Flowey. But their still Megalomaniac needs is their down fall. But, their are still an child, meaning they weren't bad to begin with. They were just tricked into thinking their a demon.
sans.: Sans is now an Scientist, an assistant to Doctor W.D Gaster, his step father. He is less lazy in this timeline, but is still very lazy in nature of course. He slacks off of work and sleeps a lot, due too his still present depression. He now wears an lab coat that's way too long for him, over his regular clothes. An hole in his left hand is present, corresponding with his glowing eye socket for it is an Implant that only Gaster is able to perform. The implant increases sanses magic abilities, giving the magic he has now without effecting his stats.
PAPYRUS!: Papyrus is now an low class member of the Royal Guard, wanting to capture an human to become second in command of the Guards. The reason he's in is because of Gaster vouched for him, which is papyrus's Genetic Father. He trys to act more serious than he know him for, but he is still the very same Papyrus we know and love. Not much change happened to him.
Toriel: Toriel has not changed.
ASGORE: The only thing that's different, is that he's friends with Gaster.
Undyne: Undyne was an result of an "failed" Determination Experiment Gaster and sans preformed. Gaster was trying to make an unstoppable warrior to help fight the humans, but ended up making an weak seeming 10 year old fish child. Little did he know, she had more Determination than any monster, but not enough to control the timeline. She grew up homeless, looking up to Garrison, back when he was head of the Royal Guard. She then became the Fish Lady we know today.
Alphys: The only thing that changed, is that she's an assistant like sans now.
Flowey: AKA Sunny. This is the Chara version of Flowey, wishing to gain enough power of the timeline to ERASE everything. They have more vocabulary than Flowey, sounding more smart. They are soulless of course, not feeling positive feelings. They want to ERASE monsters, humans, everything to gain revenge for what they all did to them.
Mettaton: AKA Hastapblook. He is an very shy ghost, too afraid to preform in front of anyone, even though he's good at singing. He looks up too Undyne, wishing he had confidence like her, acting as Monster Kid in this AU.
Napstablook: No change.
Muffet: No change.
Grillby: No change.
Asriel: He became tricked by Chara that humans are the enemy, soon wanting to destroy all monster kind. He became obsessed with this idea, even more than Chara. When they both became one, he helped attack the Humans with Chara, but they soon realized how sacred the Humans were. Chara tried to get Asriel to stop, but couldn't for he was far to gone of the idea that all Humans but Chara we're evil. They then soon died, but Asriel became an ghost, for none of his dust fell onto the Golden Flowers. It was collected be Toriel, who spreaded him on the Monster Flowers. Not having the Determination to become an Flower, he became an ghost/The Flavor Text of the game... And the new demon that comes when you call it's name.
W.D Gaster: And finally the one you wanna know. Gaster is the Step Father of Sans and the Genetic Father of Papyrus. This makes Sans and Papyrus Half Brothers. Gaster's wife, Lucida Comic, died while giving birth to Papyrus shortly. Gaster, grieving over her death, blames the Humans, having nothing else to blame. He made many inventions, his greatest being the CORE, the thing that powers the underground. His second invention was the "Wingding Implant." It was an eye implant that he made to make monster magic more powerful. There are two versions: Passive (Orange) and Blue (Aggressive). Hey gave Sans an blue one to defend himself when he was younger. He gave papyrus an double implant of Orange one's to make him more helpful to the Royal Guards. He gave himself both, orange in the right and Blue in the right. The monster couldn't handle an boost in both categories, making Gaster get the scars he has now. He replaces the Mettaton/EX fight, both pacifist and genocide.
I hope this is an good idea! Comment any feedback you have about this, and tell me if I should do anything else with this.
Listening to: https://m.soundcloud.com/shysiesta/shy-singsmegalo