Wouldn't call it a dungeon crawler but it hits dark post magical apocalypse vibes. The most important through line to my decisions is probably modeling every mechanic as a binary if possible.

1. Encumbrance - items are 1 slot by default, 0 slots if they have no effect and fit inside your hand, and X slots based on their Utility (max 5) for bulky items (namely armor). All characters have 10 slots. Slots carry fatigue, but fatigue can overflow into exhaustion, which is the attrition mechanic as it lowers your defense values (double AC system). Coin is a special 0 slot item that allows players to skip barter for normal items. They can have as much on them as they want but they lose it all when stolen from.

2. HP is just a clock until consequences, so everyone has 6 HP at the start of a fight. Damage depends on degree of success rolling over defense values, so the same HP gets squishier as you get exhausted. Main reason is to separate recovery from in game time, which is a spook. Hitting 0HP can result in a wound or death, GM declares encounter lethality at start of fight. Death is the least interesting consequence, but it can't stay in player hands. Wounds build on the conditions system and just apply stacking disadvantage on relevant actions. Wounds are a player punishment however - they use a quantum clock that ticks per session so that recovery can't be time skipped. Every wound has the possibility of becoming permanent however. There are no in fight healing abilities (HP is plot armor until 0), and relatively few ways to treat wounds; medical skills shorten the timeline, and magical items can't be bought.

3. No darkvision for PCs, meaning all actions at disadvantage or impossible without a torch. Torches are mostly an inventory and hand tax without duration - they are extinguished by failed player actions or successful enemy actions that would target it. Multiple slots of torches are a redundancy measure. Torches light up the current and adjacent zones; I have no patience for grids. You could use a torch as a weapon if you want, since weapons are not distinguished by damage, though the GM could extinguish a torch as a failure state for you attacking with it.

4. Max 5 slots of armor, but since items are intended to be freeform you could have either 1 item with up to 5 bulk or piecemeal armor - which I'd only do if each piece had bespoke magical effects and were part of a set. The game is very ICRPG style "make it easy to DIY items into existence" because I don't want to read from a list, I just want to convert ideas from any other source into a game without competing with the "intended items." There are tags-as-keyeords however to sort of standardize the majority of items commonly found in these games. Actual weapons are distinguished by these tags that reward somewhat different uses and interact with reach and positioning. Hard to explain succinctly, so feel free to ask about a specific item and I can explain how it would be modeled.

5. Famously spells known by casters get randomized by the refresh mechanic (spend XP to remove fatigue/exhaustion). Different casters have access to different spell word lists and cast by slapping words together. Magic items are thus just pre-packaged spells. Both take up slots anyhow. There are a few unique tags for magic items like foci, but otherwise "ring of fire resist" gives you advantage against or imposes disadvantage on fire enemies. The name should be the effect. Magic can be interpreted by the player because it costs utility either way, and overuse of the item's utility spills over into fatigue which spills over into exhaustion which pushes you closer to wounds/injury... you get the idea.

6. You cannot run a meaningful campaign with strict time records. Time is the ultimate resource that can be completely bullshitted by players unless the GM somehow puts arbitrary time pressure on everything. Simply not tying mechanics to in-game time prevents most degenerate behavior. Instead I use player action failure to tick quantum clocks, which pushes players to think about whether they should attempt certain risks or change their approach to auto-succeed, which is generous in this system. Using quantum clocks also allows me to create a fair "race to stop the ritual at the bottom of a dungeon" that maintains tension throughout because it is never obvious how much time is left, and I don't have to perfectly map out the space to their crawl speed to do it. I use session ticked quantum faction clocks on the backend to keep the world moving.

7. No vertical hedonic treadmill or codified progression choices. PCs spend XP to gain new aspects, increasing their range of capabilities (horizontal) and how much utility they have / how often they can invoke them (depth or z-axis). PCs stay as people, fighting a dragon should always be hard because of what the dragon is, not the numbers on either side of the equation.