Alright, let's see what we got, Felix! This is just some brainstorming, feel free to check both of my splotters (don't ask) to see if you can come up with anything yourself too Also preemptive apology over having too many characters to read through.
Let's start with my Spencers.
Finn & Brooke - She's someone with a ear to the rumor mill and a network as wide as the whole town - so in a way, if they haven't meant, it could be that Finn could be creeped out that someone knows about him like that, as she usually knows who everyone is on a surface level. She's a Quidditch player herself, being a satisfactory Keeper of the Spencer team but not good enough to make it to the Interschool team (and not half as much into the sport as everyone else seems to be), so they've definitely faced each other on the field before if he's a part of the Bourke's team. She's not really the rival type, she likes being on people's good side so to use and abuse the connection for any future favors she may need and vice versa. Though, she likes testing people's reactions at random, so, beware. Perhaps she's gotten on Finn's nerves before like that?
Finn & Gwen - She's not enrolled in the school proper just yet but she does occasionally wander into school grounds (and is partially responsible for the recent prank on the Spencer House's showers, alongside with her sister). She's a new arrival to Australia as of mid August and is looking to meet new people and explore the town - what's better to accompany this loud girl than a loud boy in that adventure? As well as get into trouble, of course.
Finn & Amber - Both Quidditch nuts, both as loud as they come. Amber has made a bit of a name for herself in terms of having the innate ability of befriending everyone under the sun, and there could also be a potential rivalry here of sorts - she's the Spencer & Interschool Seeker, after all, and always brings her best to the Quidditch pitch. It's also likely that they've run into each other before as she visits the Bourke Common Room a lot out of visiting her cousin Samantha and "hanging out" with Travis Franklin.
Finn & Laurie - A remarkably calm person for a Spencer, Laurie's frequently dragged around by Gwen (and sometimes Max) to play pranks on unsuspecting people, although she's rarely motivated to do so on her own. She's very permissive when it comes to people so there's no reason why they'd be on bad terms, and I'd go as far as saying they could be buddies - she's new in Tallygarunga as well, although she's set to leave at the end of the school year since she's a Durmstrang exchange student.
Up next, Flinders!
Finn & Ian - While they probably wouldn't have ran into each other normally, Ian is the type to go out of his way and help people out. It could be Finn has been one of such targets before, and had to deal with a happy-go-lucky Dhampir watching his back for better or worse.
Finn & MJ - MJ is the younger sister of the Bourke Quidditch Team's Captain, Aspen Clarke. As such, she goes to watch practice on occasion. If Finn is in the team proper, there's no way that they wouldn't know each other; in fact, he'd know to go get MJ in the event Aspen gets mad at something due to her uncanny ability of calming her sister down. She has a bit of a reputation to her of being a loony (unsubstantiated) due to the Clarke's father being a graveyard keeper and MJ being the quiet type, it's up to you whether Finn shares the sentiment or knows better.
Finn & Meadow - Since Finn likes to get in trouble, it could be he often gets injured. Depending on how he achieves these injuries, he might not want the school staff to know... And that's where Meadow comes in. A prodigious healer, she's willing to heal pretty much anything within her scope of abilities under the table without asking questions, for a fee. She could prove to be quite the useful ally in his adventures that way.
Now, Finn's House, Bourke!
Finn & Sam - Samantha is a fellow fifth year. She's a very social individual with plenty of kindness to spare, and being altruistic as she is, it's likely that she took it upon herself to show Finn around once he transferred to Tallygarunga simply because they're in the same House. Plus, if he can resist the Veela charm perpetually surrounding her and doesn't act on it, it's likely that she can potentially see him as a good friend. She's the daughter of a legendary Quidditch player as well, even if she doesn't play it herself.
Finn & Dani - She recently left campus and moved in with the Estrada family, but otherwise Danielle still spends a whole lot of time on campus. She's a fan of Quidditch but due to personal issues she doesn't compete nor go watch the matches. It would be nice to have someone to talk about it with every once in a while.
And finally, the best House, Sturt.
Finn & Renee - There's no way she wouldn't get under Finn's skin (and that's a theme you'll notice with my Sturts, as you'll soon come to realize). Nearly an anarchist and a bully, she spites others for her own amusement, seemingly having no concern for potential consequences. It's possible she's targeted Finn in the past due to her apparent complete disregard for her own survival.
Finn & Theo - Theodore's quite the introverted young man but he'd be lying if he said he didn't get a kick out of making a fool out of others. He enjoys mentally running circles around people and making a fool out of them due to a disdain of humanity, save for a select few people (with whom he's still a bit rough towards anyway, such as Lexi Stewart and Gideon Sharpe). If he ever came across Finn, beware.
Finn & Chloe - The one harmless Sturt that I have. She is Lexi Stewart's older sister and generally a bean, if a bit too robotic for her own good. She sees the world through a very naive lens of cooperation and 'live and let live', having a strong distaste for competition. While Quidditch isn't exactly her thing, she's a popular go-to for students whom are looking for free tutoring in nearly any subject.
Finn & Lisa - She's the resident overachiever. Being the Captain of both the Sturt & Interschool Quidditch teams, there's no way she hasn't come across Finn before if he plays; not only that, but given that she's romantically involved with the Bourke Quidditch team Captain, there's a generally decreased morale for both teams due to their disapproval of the fraternization (which the pair doesn't really care for, as they're particularly competitive against one another). She's not the nicest person in the world, placing results above all else, and unless he's a friend of Aspen or MJ she won't pull any punches when talking to Finn.
Finn & Rachel - Two fellow Irish people! Not only that, but Rachel can give him a proverbial slap in the face in the sense she's been a Gryffindor in the past (although, if my math is right, they completely missed each other in Hogwarts due to an age difference). She's the Sturt/School Quidditch team Beater, so like with everyone else I have who plays it, it's likely Finn's played with/against her at some point. It's possible that they don't get along too well, at least on her end, since she has a bit of a sharp tongue - too much so for her own good - and seems to put up some sort of wall between herself and other people, though she's occasionally spotted following Lisa around.
That's all for students! Let's see people outside of school.
Finn & Ashley - If Finn boards, it's likely he wants to have technology with him. If he does, he needs to have it all magic-proofed. That's where Ash comes in! She leads a business directed at providing that service, and if Finn is a recurring customer she may give him a discount or three to thank him for his patronage.
Finn & Jamie - Jamie's the current Australia National Quidditch Team Seeker, and he's moved back to Narragyambie recently. It could be that Finn runs into him at one point or another, which might be fun due to their share adoration of the sport.
Finn & Caroline - Caroline used to be a Chaser to Melbourne Manticore, a team in the Australian Quidditch League, until a recurring injury sidelined her permanently. Much like Jamie she resides in Narragyambie, and if Finn knows his stuff, it could be he recognizes her.
That's all I got for now! Which is... a lot? But I'm willing to hear out more ideas, and even stuff for potential threads.