For me even though it was rocky the whole way through Finn's arc was solid but would definitely benefit from some changes which I'll autistically get into now.
First off: LET HIM FUCKING GROW UP! This is one of the most glaringly flawed things about Finn, the show is a coming of age tale about a boy becoming a man but the creators are either unable or unwilling to show the kid actually physically growing up; at most they SOMETIMES draw him taller which is just not enough. Have him grow, change the design, give him new outfits or some shit, eventually have his hair showing all the time just to show he's physically maturing alongside his personality and voice, it's not hard. They eventually gave him a robo arm which was appreciated but that's not enough.
Two: DON'T MAKE HIM SHIT AT FIGHTING! When 16 year old Finn seems like he's a quarter of the warrior 12 year old Finn was something's gone wrong. I don't know if it was because the show got lazier and having Finn job was easier to write or they eventually became disinterested in Finn chopping stuff up, but this makes him seem like a failure as a hero when his skills are just degrading over time. It does not feel like he's maturing into a better hero and person, feels like he's sucking at his job.
Thirdly: CHANGE UP HIS RELATIONSHIPS! As a person grows up their relationships with other people should change, and while Finn's did evolve they never did so as drastically as I think they should have except for maybe Ice King who had great development with Finn. Jake was always good but got stagnant after a while so spicing that up would've been appreciated. The way they handled PB was so screwed up in 200 ways and really should've been handled differently with Finn reaching a point like at the end of the finale where she respects him on her level like 3 or 4 years ago at least.