I can tell you something that is working remarkably well for me, and entirely by accident: I had a sketch of a campaign world I wanted to build, but only 1 small area that was solid. It languished for a couple of years.

Then I got introduced to a website where people publish serial novels (Royal Road).

A few months later I had a bunch of story ideas bouncing around in my head, and a particular combination was demanding most of my attention, so I started writing it. I'm a bit of a 'panster', so I just started with my opening scenario and wrote.

But I found myself needing information for the world around them really fast, and then realized I had a solution: It fit perfectly in the campaign world I had started. This means that to keep writing the story, I had to keep filling out details that could be added to my campaign website. I also have two other story ideas I have started to sketch out that help fill details on other continents.

In short: If you can write a story in your campaign world, then answering questions around that story will give you campaign details.

Important note: The story you write IS NOT and WILL NOT be the adventure your players go through. Ditch that idea immediately. The story is its own thing.

Oh, I like this much better than mine. My brain got stuck on the specifics of using a real gas.

Alright, so the first thing is that Carbon Dioxide is very, very noticeable as an immediate suffocating feeling. It is the thing that your body detects when you hold your breath. But if there is enough in the environment to affect you, those situations are generally instantly lethal. Slow build ups seem to be rare.

Carbon Monoxide is the sneaky stuff. You probably won't notice it except to slowly feel lightheaded and have your thoughts go fuzzy/delusional etc.

Methane might be easier to run: It can make it harder to breathe, but it also goes 'boom'. If there is a small enough amount of it in an open area, you might survive the boom and then would be able to breathe when fresh air rushed in.

I'm guessing that this is D&D 5E?

I'm going to regret posting this on my discord where my GM can see it, yet I am going to do it anyway...

I'm going to copy my reply from your cross-post to the PF2E instance as this one seems more active overall, and maybe someone will have some more thoughts or insights.

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My first thought is to make sure you start off with Amped Shield and use the block each turn if you would get hit, that gets you three turns of reducing the damage of one hit each turn. And buy scrolls / a wand of Mage Armor. You do not want to take up a Spell-Known slot for it.

Hmm, Automaton has reinforced chassis, but unless you were already planning on having a strength of 16+, that seems less than ideal.

A human can just simply grab armor proficiency at 1st level (Via General Training). If you really want to commit, spend your first general feat to then upgrade it to Medium armor. Watch that Strength requirement for armor though. You might be better off getting the toughness feat for that general feat. Ouch, just realized proficiency does not auto-scale from this feat.

Spending two class feats to get Mountain Stance seems expensive (martial Artist archetype -> Mountain Stance), but does stack with Mage armor.

1 class feat for Sentinal dedication gets you light & medium proficiency up to Expert.

You could be a Well Spring mage, then no one will want to be near you in case you blow up! (just kidding … or am I?)

That’s all I can think of at the moment.

So, this gaming group started a Me, My Wife, Friend A, Friend B, and a couple that friend B met more recently.

For various reasons, Friend B dropped out about the time I started running a new game for the group. A little later we decided we wanted to slowly increase our numbers to buffer the ability to play if someone was missing.

I picked up a player from Reddit who had never had a chance to get into a PF2E game, all his attempts had met with DOA groups and I wanted to improve his experience. Later on, I picked up another player from Royal Road, she and I both publish serials there and follow each other's stories, and started chatting.

Edit: Oh, and all this is online, excepting the couples none of us live within convenient distance from each other. Though ironically, the other couple moved last year and are now only two hours away from us.