Hey I'm the guy who posted that, nice to see people enjoying them even if they aren't technically Finnceline.
I'll post more for people>https://www.fanfiction.net/s/8260495/1/Crimson-Temptation
Short friendship fic, Marcy hurts Finn and they gotta deal with it. Good, short read with a mediocre and unfinished sequel.>https://www.fanfiction.net/s/9681890/1/A-Life-to-Serve
AU, Finn is a slave Marceline gets, they fall in love and shit. Good read but kind of sterile prose as I look at it in hindsight. >https://www.fanfiction.net/s/10254069/1/Addiction
Very interesting and original read where Marceline is basically an evil bitch who forces Finn to fall in love with her. Fascinating, original, and well written.>https://www.fanfiction.net/s/10913273/1/Darkest-Knight
Pretty by the numbers but still good read, with a Finn characterization I like a lot. Don't get too attached though, even though I think he's still around the author hasn't updated in like 2 years, might be dead in the water.
These along with The Last Human and Finn Grows Up (which have already been posted) are the only goof Finnceline fics I've seen, but we can always hope for more.