Love me some oneshots. I'll include a few short story collections I'd personally recommend as well.
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Rengoku no Ashe, by Spy x Family's Endou Tatsuya.
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Kero Kero/Piyo Piyo, which is pretty much all of Mizukami's oneshots from before he started making full-length series (which was a long time). Special shout-out to the best one,
Walking the Void.
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The Young Maiden in Love is a Carnivore.
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Nude Model, by Blue Period's Yamaguchi Tsubasa.
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Terrarium in Drawer and
Dragon's Seven Adorable Children, two very good oneshot collections by Dungeon Meshi's Kui Ryoko.
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Shiro Ame, by Abe Yoshitoshi.
- I could recommend any Dowman Sayman oneshot, but my personal favorites are the collections
Nickelodeon-
Melancholia. Many of them are interconnected though, and Melancholia has an overarching "plot". Might not be quite what you're looking for, but strong recommendation regardless.
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Goggle and
Koohi Jikan, by Undercurrent's Tetsuya Toyoda.
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Precious 4p Stories and its
sequel, fluffy 4 page long romance short stories written by various popular modern mangaka.
- In a similar vein,
Akai mi Hajiketa.
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Final Girl, by Sakai wa Nemutteru's Kokikuji You.
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Latin and
100 HandRed. Been a big fan of Takabatake Enaga since the enjoyable mess that was Godspeed. It's a real shame he never got the chance to show off his skill with a proper full-length series, but his oneshots are quite fun.
Shoutout to
@Halo for recommending the peak Tsukumizu doujin.