How do you set a tone in a game?
Call of Cthulhu has a sanity system. It was one of the first games I encountered where there was an attempt to address genre conventions. The question of how successful this system is...that's been very debatable over the years. The more infamous incarnation of the game "Dread" uses a Jenga tower. "Dogs in the Vineyard" uses a core mechanism that is all about escalation, leading to higher drama and more intensity.
But generally I've found that most system mechanisms really don't reflect genre conventions, they typically don't add flavour even though I can sometimes see how a designer has attempted to enhance flavour through such mechanisms. I don't claim to know every game system, nor even half of the games that are out there, some games might have mechanisms that really inform genre conventions well, but I just haven't seen them.
Instead, I find that most genre and flavour elements are introduced by players and the GM, often outside the rules written on the page. I think this might be the reason why a lot of players around the world develop home brews and hacks, they're trying to put into writing the kinds of things they do at the table to get a specific feeling in their stories. Not all hacks and home brews are about this, some are just exercises in mediocrity where someone has a tiny idea to change an existing system, or renames an element of an existing system then claims it's revolutionary.
I also find that it's more common for flavour and genre conventions to manifest through the flavour text in a game. People home brew settings to attach to existing systems; the good ones offer lots of hooks and suggestions on how to use those hooks to generate the kinds of stories that match the feelings they have for their world; the bad ones are pastiches of existing settings, where it's generally expected that a player will understand those existing settings and the vibes inherent in them to understand how to combine the ingredients to get the desired output. I like the flavour ideas in "Planescape", "Dark Sun", "World of Darkness", "HōL", "Warhammer" (Fantasy and 40k), "Legend of the Five Rings", all settings dripping in flavour... all settings where the mechanisms of play don't necessarily fit the flavour of the setting unless liberal narrative interpretation is applied regardless of those rules.
Lots of issues, lots of fragmentary ideas. I know that attempts have been made to address these concepts, but again I think those attempts have been pretty hit and miss.
I've been looking at the idea of scarcity to reflect a post-apocalyptic vibe. I've mentioned it a few times already, you roll on a table and there's a chance that the item may disappear from the table and never be able to be found again; the rarer the item the higher the chance that this will be the last item of that type able to be found.
I'm looking at vague prophecy from the past to enhance the mystery of the setting. I might be drawing elements of this from the Malifaux RPG "Through the Breach", where characters lay out a tarot spread wher each card contributes a fragment toward the prophecy... I toyed with this a few months back when I generated the "Tom Waits Oracle", but I think there's more potential here.
The last thing that I'm thinking of adding is a sense of wonder...not darkness, not grittiness...there are too many games like that already, and as mentioned previously they often don't get the feeling right through the mechanisms of play, instead they get ideas across through the fluff amd flavour text.
The rest of the game will follow the simple system I've been describing in recent weeks. I don't want too many flavours conflicting or confusing the overall tone.
They are a few small steps, but there are a lot of others that might contribute overall to a certain feel of a setting.