A persistent world, lots of interesting things to do, and players decide which way their characters go. And the core activity is solving mysteries; some mundane, some turn out to be more than that. Character stables are recommended, but not mandatory.
The question is, what kind of set-up allows a sufficient amount of mysteries while keeping the game world credible?
- Cthulhu is the classic. I am not quite sure I want to live with the baggage and there is also a classic problem: how to have mysteries come in, one after another. Even if the background mystery ends up being Cthulhu, it needs a stronger concept for the characters to stay together.
- An organization that investigates mysteries, in the style of Delta green or whatever the group in Väsen is called, would work just fine.
- A sufficiently large city would allow going with private detectives, a mystery cult or some similar group.
- Some kind of school-type institute, with the characters being children, could work in a local game.
What is the background mystery? It is likely the metaphysics of the game world or something that is about to break it. Cthulhu, the remnants of folklore, serpent men, etc. A unified background mystery is probably better than a disjointed soup of everything possible mixed in, though that has its pleasures. The actual mysteries in play would be a mix or ordinary cases and things with connections to the background mystery.
A third matter is the rules system. Pretty much any traditionalish system would work fine. Anything with failing forward is dangerous, as it creates a mechanism for the game master to take away the mystery-solving from the players. Same with some Apocalypse world -type knowledge rolls.
- DIY: a skill system that allows separate types of characters, plus some other mechanical complexity, maybe. Simple and elegant.
- Miseries and misfortunes, with the failing forward removed, would work and the magic system would be a good fit. Finding players might be challenging.
- Call of Cthulhu or some other traditional system would work.
- But please, no Gumshoe, the antithesis of sandbox mystery solving gameplay.