Most of this is not canon anymore, so pay no attention to this post.
The monsters had been trapped underground for years. Eventually,the monsters settled down, though many still wait for another human to fall. The monsters had children. The older monsters retired and the children started their own lives. But one day, a human fell. What will the monsters do? Will those who stay Determined attempt to harvest their soul? Or will their own thirst for Justice in the world set the monsters free?
Differences from original game:
1. Frisk fails to let the monsters free. They still continue to live Underground, as they don't have anywhere to go up on the Surface.
2. The main characters have been replaced by their children (mostly).
3. The older monsters barely appear, but do live.
4. Asgore doesn't fight you at the end, instead you face many monsters during the game who attempt to harvest your soul (at the end, you are tasked with breaking the barrier yourself with the six souls, who are still there), and you end up fighting either Photoshop Clovwer or The Ultimate God of Hypermercy.
5. The player's name is Ash, and they have a SOUL of Justice.
Siren: Captain of the royal guard (Papyrus was so proud), but wants to make music with one of his cousins, Jasperblook. Still talks to Clover, but they were never close and have never really have been.
Jasperblook: Napstablook's son (don't ask, not even he understands) who is exactly like his father, but has a bit more self esteem and sells his own music that he composes.
Oliver: Sentry. Extremely lazy, but if one pays attention, they can see the tenseness of his shoulders and how he always casts looks around. Enjoys making puns (Basically the same as his dad, Sans, but with Toriel's gentleness).
Anemone: Second-in-command of the royal guard (appointed there without even realizing it). Enjoys drawing during her free time, but is extremely tough. Best friends with Talya.
Talya: Royal scientist. Technically not Alphys' child, but was like her sister to her. Shy, likes fantasy novels. Best friends with Anemone, but doesn't feel like she deserves to be friends with such an awesome kid. 30 years old.
Clover: Introduces the player when they first fall. At first seems like a nice guy, but then turns into a jerk and (seems) to attempt to kill you (actually doesn't want to, but is driven almost mad from loneliness of his friends giving up on him). At the end of the game you can rescue him and remind him of his place in the world. You have the choice to give him half your soul, which will give him the body of a goat (kind of looks like Asriel). If not, he will revert back to flower form and go to the surface with you all, staying with Frisk and Flowey.
Frisk: Lives in the Ruins with Flowey, Sans, and Toriel. Saves you from Clover and brings you home and heals you. You are introduced to the others, and Flowey attempts to attack you, which causes you to flee. You end up outside the Ruins and, hesitantly, continue on. Appears a few more times later.
The Royal Guard: Except for Siren and Anemone, doesn't appear much. Full of dogs and consists of: Woofiliam and Dogelope (twins), the Annoying Dog, Barky, and Mediocre Dog.
River person: Mysterious. Tends to tell you 'Beware the man who-' then stops in the middle of their sentence and shout TRA LA LA, then go silent before trying to tell you again.
Casper: Daughter of Monster Dad (Monster Kid). Runs into you at Waterfall and decides to accompany you without telling her parents.
Genocide: Ash (the player, you) kills everyone and ends up fighting Oliver. Oliver uses his father's Gaster Blasters (as he was killed by you already, since he is older and even lazier now) and attempts to make you pay for everything. If you kill him, you can go back to the surface and have the choice of taking the SOULs to set them free.
Pacifist: Ash (the player, you) Spares all and makes friends with everyone they can (EXCEPT JEREMY, JERRY'S SON. HE A JERK.). They can choose to leave the Underground, by themself, or stay with them, as the SOULs had been stolen by Clover (turns into Photoshop Clover) and set free already, sadly not destroying the barrier.
True Pacifist: Ash (the player, you) Spares all, but when they reach the Judgement hall, right in the middle of Oliver's dialog, Frisk intervenes and stops him from judging them, as they all there as proof that the player isn't Genocidal. As the main monsters, Frisk, Anemone, Siren, and Talya, arrive and start talking to the player and to each other, Clover appears and the monsters are encased in his vines. His (former) friends, recognizing him finally, beg for his Mercy and to be put down. Clover refuses, claiming that they all stopped talking to him (except for Siren, who didn't recognize him and wasn't even that close to him in the first place) and his own PARENTS forgot about him. Frisk attempts to apologize, but Clover laughs, and, after a bright flash of light, is gone and replaced by another goat boy. He has a chain flower necklace, green gloves, brown pants, and a green shirt. The goatboy claims that he is Clover and turns into 'The Angel of Darkness', who attempts to kill the player to take over the Resets again, to go back to the time when he was appreciated by his friends and loved by his family. You have the choice to SAVE him. If you SAVE him, the lost SOULs of your friends appear. Once you SAVE them, they will use their Determination to help you earn Justice and SAVE Clover. After this, he will turn back into the little goatboy and start to cry. He says that he just wanted to be with his friends and family again. After you comfort him, he will break the barrier and turn back into a flower. You have the choice to give him half your soul, which will turn him back into the goat boy. You have the choice to bring him with you or not. You and the other monsters make it to the surface and rejoice. At the end, you will have the choice of going with your new friends or not. If not, than a picture will flash that shows the player with glowing red eyes. If yes, than it will show a kitchen with all the monsters around a table, along with the player, and they are having a feast.
Lost SOUL Oliver:
"gIve Up alReadY. Is iT rEAlly wOrth It?"
“whAt’s The pOiNt?”
"Thanks kid, and don't give up. We're rooting for ya."
Way to SAVE him:
Flirt: THE LOST SOUL IS FLUSTERED BY YOUR COMMENT, BUT RECOGNIZES THIS ACT SOMEHOW.
Pun: THE LOST SOUL CHUCKLES AT YOUR PUN, AND SEEMS TO REMEMBER A TIME LIKE THIS. THE OTHER SOUL IS LESS THAN AMUSED.
Papy's Spaghetti (from inventory): THE SOULS RECOGNIZE THE SMELL OF THE GROSS SPAGHETTI, ONE SEEMS TO ENJOY IT MORE THAN THE OTHER. (If used at first, than only one more ACT is needed to save both Oliver and Siren)
Hug: THE SOUL DOES NOT KNOW WHAT YOU ARE TRYING TO ACCOMPLISH, BUT THEN IT REMEMBERS!
Lost SOUL Siren:
"cAPturE ThE huMan!"
“tHAt WiLl mAke dad..”
"WAIT A MINUTE! YOU’RE MY FRIEND! I WOULD NEVER HURT YOU!"
Ways to SAVE him:
Flirt: THE SOUL IS EXCITED THAT YOU ARE FLIRTING WITH HIM, AND FEELS LIKE THEY'VE SEEN THIS BEFORE. THE OTHER SEEMS JEALOUS, THOUGH DOES NOT KNOW WHY. (This will work on both, but will only count as one of the ACTs).
Dramatic pose: THE SOUL POSES WITH YOU, AND SEEMS TO RECOGNIZE THIS ACTIVITY.
Papy's Spaghetti (from inventory): THE SOULS RECOGNIZE THE SMELL OF THE GROSS SPAGHETTI, ONE SOUL SEEMS TO ENJOY IT MORE THAN THE OTHER. (If used at first, only one more ACT will be needed to SAVE Oliver and Siren)
Sing: THE SOUL RECOGNIZES THIS ACTIVITY AND JOINS IN. IT REMEMBERS!
Lost SOUL Talya:
"i'Ve FaiLEd You. yOU MuSt hAtE me."
“EveRyoNe muSt hAte me…”
“I jUst scRew uP..”
"Wait a minute! That’s not true! Y-you d-don’t hate me! You’re my friend!”
Ways to SAVE her:
Complement: THE SOUL IS EASILY FLUSTERED, BUT RECOGNIZES THIS COMPLIMENT FROM SOMEWHERE.
Talk: THE SOUL ENJOYS JUST HAVING A NORMAL CONVERSATION, ESPECIALLY ABOUT FANTASY, AND HAS A SENSE OF DEJA VU.
Dodging her attacks: THE SOUL WANTS YOU TO KEEP DODGING THE ATTACKS, BUT DOESN'T KNOW WHY.
Mercy: THE SOUL REALIZES THAT IT HAS MET YOU BEFORE. IT REMEMBERS!
Lost SOUL Anemone:
"HarVeST tHeiR SOuL!"
“ALl huMaNS OnlY KiLL!”
“i mUST Get tHEM..”
“Hey, wait a minute! Not all humans are bad! In fact, you’re pretty cool!”
Ways to SAVE her:
Fake hit: THE SOUL IS GRATEFUL THAT YOU DIDN’T ACTUALLY HIT THEM. IT SEEMS TO RECOGNIZE THIS ACT BUT DOESN’T KNOW WHY.
Flirt: THE SOUL LAUGHS AT YOUR CHEESY PICK UP LINE. THE PICK UP LINE REMINDS THEM OF SOMETHING, BUT THEY DON’T KNOW WHAT.
Compliment: THE LOST SOUL IS HAPPY YOU LIKE IT’S SHIRT. IT REMINDS THEM OF SOMETHING, BUT WHAT IT DOES IT CANNOT TELL.
Mercy: THE LOST SOUL REMEMBERS THE ONE TIME THEY HAD BEEN SPARED. IT REMEMBERS YOU!
Lost SOUL Frisk:
"Stay Determined, always."
Ways to SAVE them:
Flirt: THE SOUL HAS HEARD THAT ONE BEFORE. IT FEELS LIKE IT'S EVEN SAID IT BEFORE....
Butterscotch pie: (If in you inventory) THE SOUL REMEMBERS A HAPPIER TIME.
Mercy: THE LOST SOUL HAS A SENSE OF DEJA VU.
Remember Justice: THE SOUL REMEMBERS YOU AND THIS FEELING OF DETERMINATION.
This is an AU I made up, please do not steal the idea. I didn't want to make apage for it just yet, just wanted to see what others thought of it first.