Flatland (or, its full title, Flatland: A Romance of Many Dimensions) is an 1884 novel by Edwin Abbott Abbott that satirizes Victorian culture and explains the concept of dimensions. It imagines a two-dimensional world populated by flat lines and shapes.
The Flatland fandom is very small, but not non-existent. Gravity Falls crossovers are popular because Bill Cipher is a triangle. The fanfiction Flat Dreams by PengyChan probably started this trend.
Flat Dreams  by PengyChanFandom: Gravity Falls/FlatlandDate: 20 Feb 2016–13 May 2016Length: 53,940Status: CompleteGenre: Crossover, Backstory
"This is based on the premise that Bill’s dimension was Flatland - a two-dimensional, strictly-regulated world inhabited by geometric figures, where each one’s place in society is determined by the number of their sides. The whole story is told through the memoirs of A Square, who is shown the existence of a tridimensional world - only to be imprisoned by authorities to keep him from speaking of it. Said memoirs are exactly how Bill will find out about other dimensions, too." — Author's note
on the first chapter.
On the Nature of a Side  by UrsulaKohlFandom: Possession - A. S. Byatt/FlatlandDate: 21 Dec 2017Length: 2,315Status: CompleteGenre: Crossover, Epistolary
Written for Yuletide. AO3 summary:
"Once one has uncovered a set of nineteenth-century letters, one is forever an expert upon such discoveries, even if they entail the mathematical fantasia of a person such as Agnes Hart, niece to the famed educator Edwin Abbott Abbott."