Alrighty~ A while ago, when I was starting the Suginami sprites, I had a few ideas going around in my head. I doubt they will fit with what you are planning, but hopefully it can inspire you a bit~ ^^
So the big thing about Little Busters is childhood. Every character has an event in their childhood that directly impacted the happenings in the Litbus world. To keep true to the source material Suginami’s problems and dreams should be the result of something important about her past.
My idea is that Suginami was originally from a small quiet town in the countryside. Her family weren’t particularly well off finance-wise; her father - who worked as a landscaper - was the only source of income, but that was fine. A lot of people had left the town with their children in hopes of giving them a promising education from city schools.
The Suginami family were dedicated to the town. Mutsumi’s father had essentially shaped and designed the whole town… it was his life’s work. Mutsumi’s mother wanted to move to the city in hopes of a prosperous future.
While the two parents lived with conflicting ideas of the future, the town shrunk drastically. When Mutsumi was twelve years old the town school was closed, and her mother decided that it was their final chance to leave. They had to leave… with or without the father’s compliance.
Mutsumi and her mother moved away, and Mutsumi never saw her father again. Eventually they were told that he had passed away.
Mutsumi entered the local Junior high school a few months after the school year had begun, but she was left ostracized almost immediately. It wasn’t long before the snickering comments of classmates began… “She’s from the countryside, right?” “Eeeh, really? What’s she doing in the city?” "Keh, maybe she escaped from the pig pen!"
Then along came a group of three girls. A brown haired tomboy approached Mutsumi with a big grin. "It’s no fun being alone. We’re gonna do some Karaoke. Wanna come?"
Mutsumi shied away from the offer, but was eventually persuaded by another member of the group - A dark haired girl with a bubbly atmosphere around her, like an older sister.
The plan was, if the group of girls stood up against the bullies, then everyone would be impressed and reward the group with praise. Befriending Mutsumi was a good reason to get involved. Surprisingly it worked. The group called out the bullies, and managed to stop it before acting recklessly. As long as the group stood up for Mutsumi they felt important… Until Highschool. It was all quickly forgotten during the transition to highschool.
The group had no one treating them as important figures. They needed to boost their reputation and become people that everyone looked up to!
There was a girl who constantly achieved high grades without studying at all. That wasn’t fair for the rest of the class… but her name was renowned. Bully her or accept her? Both would work out… right?
Then it segues into the events explained during Litbus. The plan failed and the group continued to sour.
Then you’d have all of the present-day stuff going on. The only future events I really thought about after this was the involvement with the secret of the world’s influence.
Riki learns all this and uncovers Mutsumi’s wish to go back to the countryside and say farewell to her father. From the memories Mutsumi still remembers, Riki is able to narrow the town’s location down to two places, one of which he is familiar with. There was a countryside town where Riki and the rest of the Little Busters held a summer excursion… the house of grandfather Natsume.
Riki goes and tells his plan to Kyousuke, and asks if he would bring the rest of the Little Busters, but Kyousuke shoots down Riki’s ideas and tells him that visiting the town would be pointless.
Naturally, Riki goes anyway… and realizes something strange. Grandfather Natsume vanished. Why didn’t Kyousuke ever mention it?
Then they go down the rabbit hole. Riki bombards Kyousuke with meaningless questions that seems meaningful at the time - Kyousuke uses Lennon to try and make Mutsumi return to her group and leave Riki alone - Mutsumi has no idea what to do in the current situation.
Soooo, hopefully at least one of those ideas is inspiring in some way~ There’s probably a bunch of plot holes though… It’s been a while since I read Little Busters =w=