An outline for a one-shot adventure with the situation, what the characters are, the aim of the one-shot, events during it and an ending.
The PCs are all monsters of the same type who must ransack an enchanted magical laboratory. The mood might be... surrealThere is an attractive ranger who offers them aid. they are the parent of a character. The characters start at a disrupted funeral. At some point they encounter an untrustworthy NPC. If things are going nowhere something goes missing.It finishes with feasting in an inn.
The PCs are scarred barbarians who must survive a slaad apocalypse. It might involve elemental air.There is a famous fighter who offers them aid. they grew up with a character. The characters start with the loan or gift of a powerful magic item. Partway through they encounter a sand storm. If things slow down someone tries to steal from them.It finishes with a reward of another quest. Possible sequel... an NPC from the adventure hires the PCs for something totally different
The PCs are all undead who must sell a haunted helmet. It might involve lycanthropes.There is an angry monk who offers them aid. they are part of same religion as a character. The characters start with an escaped monster. Early on they spot foes who are unaware of them. If things slow down they find a written note.It finishes with a character or villain coming back from the dead.
The PCs are scarred warriors who must negotiate with a crazed knight. The mood might be... grim and grittyThere is an infamous old dragon who works against them. The characters start running away from something. Partway through they spot foes who are unaware of them. If things slow down a new adventurer turns up.At the end there is the start of an apocalypse.
The PCs are veteran crime lords who must slay an artifact-wielding sahuagin baron. The mood might be... tenseThere is a charismatic noble who betrays them partway through. they are the distant relation of a character. The characters start in a dream sequence. Early on they meet a lover of a character. If things are going nowhere someone has/remembers a dream.Afterwards there is a reward of the contents of a mysterious box.
5e Encounters bundle, 11 pdfs on DMs Guild