A work of fanfiction, most often with a shared fannish language and difficult to define genre.
FFVII / FFIX crossover. General Beatrix is a SOLDIER, Doctor Lucrecia runs the experiments.
History of the Runestaff. Hawkmoon and D'Averc. The architect that Jehamia spoke of was a Frenchman, not a Boar.
Professionals (CI5) / Rats of Tobruk (WW1). Doyle's just joined the army. Logically he knows the reasons why, but no one warned him about the culture shock. The loneliness is beginning to hurt, right up until the moment he falls into a reluctant conscript's arms and sweeps him off his feet. Of the 'men who get into a fist fight together suddenly become best friends forever' trope.
Dragon Age II / Court of the Lion fusion (in flavour rather than literally). Sebastian loses faith. Fenris re-establishes it, along with the foundations of an empire. Written pre-Inquisition.
Dragon Age II / Patrick White's "The Aunt" pastiche. Where Gamlen also loved the dashing apostate Malcolm.