(just a few ideas from notebooks, so they aren't lost)
• cruise ship as a floating dungeon. "Costa Concordia", for example, held about four thousand people – this is a small town of population, packed densely into a much smaller space than a town would take; it had more than ten desks, and each of those desks could be, like a level in Moria, wracked, whacked, messed up to create different connections. Made the ship into a gargantuan predator of industry with former cruisers and staff becoming serving bodies to its self-perpetuating existence. Certainly not a new idea – as a (generational) spaceship it was done with mastery and scope a cruise ship cannot reach (no AIs to be gods, for example) – but there is something interesting to me in the idea of using real-life sea giants and their real-life floorplans, in their claustrophobic layouts designed to pack as many people as possible while keeping an appearance of 'luxury', and completely parasitic, conveyor-belt idea of 'high-life' in a cruise sustainable only by external resources, and how easy it is to imagine not a single such unique ship but a flotilla of them, terrorizing some poor coast.
It might be even done without going into overused CoC-tropes of the deep.
• Labyrinthonia – nice name but also the idea of the world infected with labyrinths: open spaces are at premium and has to be vigorously maintained, or they'd fold onto themselves with walls and grow with corridors, and then, unavoidably, monsters will appear as well. Sleep under open skies or the roof will suffocate you. Suffer no corners, build no shelter, burn the labyrinths as one would burn a fungus.
• a world where recently post-apocalyptic reality is sort is only anchored by 'weight' of human presence but otherwise fluid: the more peoples one place has, the more stable it is. I saw this setting in the dream, where it was portrayed in black and white as if an episode of old 'Twilight Zone': small wild-west-like towns are mostly in one piece, but places between them and everything that isn't 'tended' by human presence on a regular basics, tend to shift and change; old mine might become a forest, mountains turn into noxious mire, with all troubles it brings, rangers doing weekly rounds just to maintain a recognizable landscape. Curiously, this anchoring effect was not about consensus, or perception per say – people didn't need to agree on thinking about places in a certain way, neither the places they 'anchored' were a subject to their will. It looks as if a simple presence was enough, and in this post-apocalypse with its understandable lack of people, the murder or a capital punishment were considered a crime in a large degree because each presence mattered. Logically one would think big cities would be in a less trouble, but if you imagine a modern big city inhabited, at best, by quarter of population, some parts of it are bound to be weakly anchored, 'soft places' shifting into something different every time one leaves and comes back.
In the dream I dreamt there was a sense of something preventing a logical fix to the problem (which would be an increase of human population), so places were remaining disconnected and rather disjointed. I think if I am to do 'points of light' setting, this kind of setting would be something I'd use for this purpose.
• to become a mage one has to become mute and devoid of concept of language, and then to rediscover it again all by themselves; if one to become a mage, they are to make the universe to understand and submit to their personal language, and not the other way around.
On a purely game-text level this explains why it is necessary to decipher 'magical writings' for each personal spellbook or scroll: there simply cannot be a universal language of magic with this approach. It might explain why strange magical things happen to small children who speak in idioglossia, and why, as they grow, the imposition of commonly understood language destroys this connection.
• small ghola lifar mansion for Ghost Detective game, where assess to rooms changes depending on one one traverses it and what mementos they have. I thought using pentominos for a layout, partially because pentominos are more complex than tetraminos but have a number of unique combinations more manageable for a map (from 12 to 63 layouts, depending how to count unique) than hexominoes (from 35 to 216, comparatively).
Interesting idea is that that most pentominos can be transformed into most of other pentominos just by shifting or moving of a single square, i.e. in a single step. Depending on how strictly to consider a one-square shift (only a neighbouring square can be rotated or moved, for example, or any square at all) one can build a sort of range of layouts where layout can be shifted easily into another layout as a main playfield for the mystery and use exceptions that require two or more shifts as secret exits or revelation chambers of sorts.