I quite literally have a folder called 'concepts' where I toss notes with ideas I have but don't have the time, wherewithal, etc. to develop at the time. Sometimes I come back to them. Sometimes I delete them because they were stupid. A few get promoted to WIP.

Samples:

• The Fun-stage Situation: A guy shows up, captures some people, maybe a family, and forces them to do fun things under penalty of death.
• Where's Waldo and Carmen Sandiego travel the world committing crimes
• Lifecycle: Two ships crash into each other, they merge materially and as crew, and grow by devouring other objects but it eventually grows unmaintainable and 'dies.'

I've learnt that if you have a cool idea that doesn't fit in your primary world, just create a multiverse :p

A campaign where characters are in a quantum leap type scenario and don't realize it going in. First session is normal. Generic D&D setting and basic quest like "Cultists of the Red Hand have kidnapped some people from a nearby village and King Harold has tasked your party with rooting them out from the nearby abandoned castle."

Next session, change the setting entirely and without warning, but keep the general details the same.

"Sheriff Harold has deputized your posse to round up the outlaws of the Red Hand Gang which has kidnapped several townsfolk from the nearby settlement. They're holed up in the old abandoned mine."

Next session, same thing.

"Grand Judicator Harold has contracted your team of specialists to rid the sector of the Red Hand Space Pirates who are holding a group of colonists hostage at a nearby abandoned station."

Keep changing the setting/genre every session, have the PC's be the only ones aware of the changes. Main storyline is figuring out who/what/why is causing the reality shifts and trying to get back to their own reality while still fulfilling the original quest objectives.

Settings start out predictable (Fantasy, sci-fi, western, etc) but slowly grow more unhinged (circus-world, cells in a body Osmosis Jones style, otherwise normal world but everyone is a fish in individual tanks piloting mechs, etc.)

My brain does this neat trick of coming up with ideas when I cant write them down (like driving) and then forgetting them forever.

I have been thinking about a TTRPG character idea inspired by the Skyjacks world created by James D'Amato.

A religion based on an existing but absent god. Immortal Angels were the only visible, but rare, part of the Host, but they all apparently died when the god stepped away from the Throne.

They reanimate some long time later without direction from management. The hierarchy of the church has been using pieces of apparently-dead angels as relics, parting them out to churches and temples. When the angels reanimate, they are scattered in pieces and suffering immensely.

The church sees the relics twitch or make noise, and view it as miracles signaling the return of God. Because the miracles bolster the power of the church which has been so long without apparent divine favor, they don't consider giving up their relics.

One angel reanimates where they were entombed by the old-fashioned priest of a far-flung village. Together they embark on a quest to take back the relics of the corrupted church and get the band of angels back together. With enough angels, maybe they can convince God to come back to the Throne.

Writing prompts saved in a text file, scribbles, drafts of images, spreadsheets to randomly generate certain information

I have a character that I don't know how to use but I love anyway. The Demon Frog Prince:

A demon prince is cursed into the body of a frog, only to be released when he receives true love's kiss. Which, as a demon and now also a frog, seems like a remote possibility. However, he is still immortal, so he lives his long life as a frog in a pond. Occasionally he is caught by predators, and even torn apart and eaten, but he always reforms into a frog.

One day, he is caught by an advisor to the king. The king has gotten into his head that he is likely to be imminently poisoned and has decided that the best countermeasure is to have a frog taste all of his food, believing frogs to be particularly sensitive to poisons. The advisor places him in a tank at the court along with other frogs, and pulls one out at random for every taste test.

This method works because most frogs are vulnerable to most poisons, so the royal family adopts the practice for generations, until one day the demon prince is selected to taste a poisoned dish. The frog doesn't die, but the king is fatally poisoned. Because of the test he is assumed to have died of a natural illness, but the advisor becomes suspicious, and performs a series of tests on the frog, ultimately discovering that it is immortal. He keeps it in his secret laboratory and subjects the demon prince to much cruelty.

At some point, a child meets the demon prince and adopts it as a pet. Might be a royal heir, might be the advisors child, maybe a child of one of the palace staff who snuck into the lab. But the child loves the frog like a child loves a pet, and one day kisses it on top of the head. The child's true love for the frog breaks the curse and frees the demon.

At this point, the story can veer off in a number of directions. I imagine the demon will have some kind of affection for the child who freed it, and maybe they become best buds and go on adventures together. Or, maybe the demon just deigns to spare the child's life. Maybe the demon and the child team up to topple the dynasty that imprisoned the demon for so long.