I have stuff to work on but I'm kind of cooking with some ideas for a Deltarune x Look Outside AU
Kris and Susie were are at school when the Visitor (or perhaps something else, but for simplicity I'll go with the Visitor for now) arrives. Kris is asleep at first and is awoken by the sounds of their classmates transforming and runs out of the room without looking outside, so they remain human. Susie gets a full look out the window, and transforms into what she thinks others perceive her to be, an inhuman monster with large claws, piercing reptilian eyes, and sharp teeth. Kris runs into the supply closet in the hallway to hide, and Susie follows them, not for any malicious reason, she just also wants to hide and thinks Kris picked a good place. The closet has been changed, becoming a long hallway, which eventually opens up into a large and strange room, with Ralsei in the center. His role is very similar to Sybil. He doesn't know how he got there, as far as he knows he didn't exist before that moment, but he feels like he's been waiting for a very long time. He also somehow knows that it will me safe to look outside again in seven days. Through the strange dark room, they can travel to other dark places nearby, which is how we get our chapter progression, and they can also use it to save.
The first place they end up is the back room of a board game cafรฉ, which stands in for Card Kingdom. There are Rudinns with cards for eyes (the backs of the cards are their eyelids) and Hathies with beating aces of hearts sticking out of their chests, and of course Pippenses with eyes in almost every pip. The first idea I had was actually a casino so I wanted King to be a card shark who became a shark themed boss, but since a board game cafรฉ is better I decided that he's still a card shark, his son just takes an interest in the games he plays, so he frequently takes Lancer to the cafรฉ to get his age-appropriate communal card game fix. Roulxs is another frequent customer who likes to flex his strategy game skills, only he's abysmal at strategy games, so when he tries to barter with the Fun Gang for food he loses every time. Jevil may be a reality-warping dungeon master. The general story is that the gang searches the cafรฉ for extra food while defending themselves from aggressive Cursed, Susie's new form is powerful so she attacks pretty much everyone she sees until they meet Lancer who thinks she looks awesome. They bring Lancer with them so they can keep him safe, leaving Card Shark where he is but promising to come back.
After going to the library (which I have not thought enough about yet to write down), they end up in a closet in Kris' home. Toriel saw the Visitor in a reflection off a glass pie dish, and her transformation primarily manifested her desire to protect and care for her children (note: this is not her only character trait, I think very deeply about Toriel as a character, however this is my best explanation for her becoming a giant huggable fluffy thing and I think it fits her mindset). She agrees to let Susie and Lancer stay over, so long as they have enough food for everyone. As for the other Dreemurrs, Asriel is currently stranded at college, and Asgore's fate is currently undecided. He strikes me as the type who would join the astronomers, but he would also be very curious about what was happening outside and probably look.
TV World is an old video rental store that the Dreemurrs used to frequent. Tenna and Kris weren't exactly friends, but they would come in to rent movies to watch while their parents were fighting, and Tenna would give them discounts. Now Tenna is a literal TV man (his ass looked outside) and is sort of like Lyle in the way that he's obsessed with the thing he turned into and he hides his body, wanting to seem approachable and cool to people. His business has been seriously struggling for a long time because of the rise of streaming and a lot of people no longer owning VCRs and DVD players, he's pretty scattered mentally and starved for attention. He has VCR and disc slot mouths and "eats" the films in his stock, magically obtaining memories of people watching those movies. He suddenly feels that he knows Kris very well and that they're close friends.