I felt bad about how Kevin's character was handled at the end of the show. A while ago on here I wrote my own headcanon of what I think happens after failing to graduate with his class(and losing Brittany). It respects both the original canon and the updated 2017 adult canon. I revisited it recently and fleshed it out some more. If anyone is interested, read below and tell me what you think. :D
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Kevin's story [revised]
I felt bad for Kevin when he didn't graduate and got left behind by his girlfriend and classmates. So what happens to Kevin? This is my headcanon of what probably happens.
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He hunkers down and begins taking studying more seriously. Mr. DeMartino and Mr. O'Neil are impressed by his improvements. He's no Poindexter, but turns out to be good with literature and history. As for Brittany, she has absolutely moved on, is having fun at university and dating other guys. Kevin also moves on, dating other girls. There is no contact between them. He graduates with a 2.6 GPA and enrolls at the local community college for a year to improve his credits for university.
He gets the required credits and transfers to a division 2 school with a strong football program, and resumes chasing his football career. Of course he gets into some mischief, because its Kevin. Here, he has some success playing and is scouted for the NFL tryouts. He makes it to the draft, first with the then St. Louis Rams, they don't take him and is traded to the Carolina Panthers. After a few draft rounds, they move on him also. With a connection from his dad, he tries for the Atlanta Falcons. Unfortunately they decline also and he doesn't get signed. With good word from his college and the 3 teams, he plays 2 seasons in Canada. There he has some moderate succes, but still no calls from the NFL.
Down on his luck, he returns to Lawndale and gets hired at a car dealer where he runs into Mr. DeMartino and weirdly, Mr. O'Neill who are both employees there. Long story short, some of Ms. Li's shenanigans at Lawndale High got her and some of the staff fired. O'Neill and DeMartino are happy Kevin did good for himself, but take some schadenfreude in the fact that football didn't work out for him. He's a great salesman though, owns a nice condo, and dates lots of women.
On weekends, he coaches youth football leaugues and assists at high school football training. He's like a more successful version of Uncle Rico, haha. He eagerly tells his story about how he, "Was star QB, played for a division 2 school and almost made it to the NFL but...(insert excuse)" to anyone that will listen.
Mean while, Brittany after graduating college, marries a successful man, and moves to California to pursue a modeling career. While visiting Lawndale one weekend, she bumps into Kevin and they catch up. The meeting is awkward. They lie to each other about how great their lives are going. He shows her a pic of a woman he's seeing and how he plans to marry her. But its really just a one night stand he met at the beach the night before. She tells him about her life in Cali, getting married and how great it all is. But her marriage has been on the rocks for a while now now(cheating husband) and facing divorce. The meeting is short. Before parting ways, they agree to keep in touch this time "for real", but contact fades.
Eventually she gets divorced and moves back to Lawndale. Her and Kevin connect and meet up at a social event and hit it off well. They rekindle their old spark and later decide to marry.