In Trail of Cthulhu, if you suffer madness you must leave the room while the other players and DM debate the most appropriate manner in which to gaslight you. They may, for example, pass notes to each other and exchange meaningful glances. Or, they could all agree that some important recent event didnt happen and will react with confusion when you bring it up. Or they could come up with any number of other thjngs.

In oWoD’s Hunter: The Reckoning, you can’t advance your Edges (supernatural abilities) with XP.

To do so, you have to gamble Conviction during the session, while using said Edges (those Conviction points gambled add dice to your pool, ideally improving your odds).

If you succeed at the task, you gain Virtues (with which you can later buy points in Edges).

if you fail the task, you lose Conviction points. If you run out of Conviction, you lose your ability to use Edges at all until you rest/meditate/recover for a full week.

Not sure if it qualifies as “conceptual” or non-mechanical to you. But I love it.

it's not particularly new but i've always found it interesting when games mechanize metagaming.

fate's point economy allows players to actively change the game by putting themselves at an immediate disadvantage. when you play to your character's weakness, you get a point which you can cash in later to modify a story beat or situation. but you can also pay other players with them to suggest actions for them.

How to jump over planning with Blades in the Dark. It's not the first gale with this mechanics but It's the most known.

Lighthearted has an interesting emotion mechanic.

At the start of a scene, you describe your current emotion. Then, when rolling you choose an emotion based on how you describe the action and the closer it is from your current emotions, the higher dice you roll. However, it'll make your emotional state shift. And if you roll your current emotion you gain stress.

While it's pretty interesting, it let a lot of room for negotiation between PC and GM which isn't always great compared to games with tighter rules

I really love Mouse Guards simple skill progression where skills advance when you have succeeded AND failed with them enough times.

Its a lot of work to deal with it, but in theory I'm a huge fan of Avatar Legends' Principles system. Basically, every playbook has an internal struggle between to extremes - compared to an Yin/Yang - and work to find their own Balance in that. The Center of this Balance starts on 0, but over the course of the game players will move towards one side or the other, to a max of +3/-3 (moving positively towards one side means you're moving negatively towards the other)

The Guardian, for example, struggles between Self-Reliance Vs. Trust; They want to protect others not just to keep them safe, but also because they don't believe they're capable of doing so. A Guardian shifting towards Self-Reliance is more cynical, while a Guardian shifting towards Trust will rely on their companions more often.

Aside from the narrative meaning, the Balance can be used for rolls; if a PC performs an action in accord to their Principles - say, the Guardian attempting to Comfort someone not out of sympathy, but because they believe they're the only ones capable of doing it - they would roll with that Principle instead of the Attribute that the Move requires. That way, if your Self-Reliance is higher than the Attribute, it might be a good idea to pitch the scene in that way.

Characters can also be called out on acting/not acting according to their Principles, or Lose their Balance (basically have a mental breakdown) if it goes past one of the extremes.

I like the fighting mechanics of Honor+Intrigue

In a gist, you have Health Point and Stances, you can « dodge » an attack by decreasing your Stance (which give you penalties) or accept the blow. When you’re out of Stances or HP you have lost