Making more creatures weak or resistant to slashing, piercing, and bludgeoning

[Generic NPC]

When playing D&D, I've noticed that it rarely seems to matter whether a weapon deals slashing, piercing, or bludgeoning damage. Barely any enemies seem to be weak to one but resistant to another, with skeletons being one of the only exceptions. For a custom tabletop system I've been designing, I'd like to make a weapon's damage type more meaningful, so I plan to have weapon weaknesses and resistances be much more common across creatures. I already have the mechanics and game balance worked out with this in mind, but I'm trying to determine a set of general rules to tell which of these damage types a creature should be weak to or resistant to at a glance, taking into account things like the creature's size, natural armor, and monster type. The weaknesses and resistances used in this system can be minor, adding or subtracting only a small proportion of damage, so a creature doesn't necessarily need to be catastrophically weak to something or impenetrably resistant to something to have a weakness or resistance.

The overall goal is that after encountering a few creatures, players will start to pick up on patterns behind enemy weaknesses and resistances. That way, when they encounter a new creature, they can make an educated guess as to what weapon might work best. "Hey, this is a new enemy. It looks like it has a hard shell, so my bow might not work so well, but your mace would. I'll help you with supportive abilities while you handle the main offense, okay?" (You know, something like that.) With this in mind, weaknesses and resistances should be logical and somewhat predictable, and not feel random.

Here are some examples of what I'm talking about, although being based purely on my "gut feeling", they might be hilariously wrong. When mapping out a creature's stats, I'd take all of these rules into account to figure out what its weaknesses and resistances should be.

Size:All else equal, a smaller creature seems more likely to be vulnerable to bludgeoning because its small frame can't resist crushing force as well; maybe it would be resistant to piercing because it would be harder to land a direct precision hit on something small.

A bigger creature might be vulnerable to slashing because its surface area and bulk make it easier to contact more matter with each slash; on the flip side, I'd imagine that it would be more resistant to bludgeoning, with its large frame able to absorb impacts and shocks fairly well.

I'm not sure what the "sweet spot" on piercing attacks would be, though, or what size might be more resistant to slashing damage. I'm also not sure whether medium-sized creatures would have any particular weaknesses or resistances, as opposed to just being a balanced "middle ground" between the small and large creatures' properties.
Natural Armor:I'd imagine that creatures with hard bodies or shells, like tortoises, beetles, golems, and skeletons, would be vulnerable to bludgeoning and resistant to slashing and/or piercing, much like how certain historical armor was in real life. I'm not entirely sure how to determine which of these should resist slashing but not piercing, piercing but not slashing, or both slashing and piercing, though. I can guess that scales would be more resistant to slashing, but could be bypassed with piercing, but beyond this, I'm not sure.

I can also guess that woody plant creatures would be vulnerable to slashing but resistant to piercing, like ordinary wood is, whereas more herbaceous and grassy plant creatures might be resistant to bludgeoning instead of piercing, much like ordinary ground plants.

Gelatinous creatures seem like they would definitely resist piercing, with their bodies just reshaping around the weapon, and I'm guessing that they might be resistant to slashing for the same reason; maybe bludgeoning would work best by rapidly reshaping their forms and thus putting stress on them.

Flat or sheet-like creatures like manta rays and ittan-momen definitely seem like they would resist bludgeoning but be weak to slashing.

I can guess that "unarmored" creatures would be most likely resistant to bludgeoning for the opposite reason that hard-shelled creatures are weak to it; the lack of armor would probably leave them more vulnerable to slashing and/or piercing, but I'm not sure how to tell which creature should be vulnerable to which (or both).
Monster Type:It feels as though bird-like and fish-like creatures might be vulnerable to piercing because its ease of reach in the air and water could catch them more easily; on the flip side, maybe bludgeoning might be less effective on them because of its often poorer reach and precision. I'm more confident in the former than the latter; in particular, birds being weak to piercing just "feels right", somehow.

I'm not entirely sure whether the average furry mammalian creature, scaly reptilian creature, or slippery amphibious creature would have notable weaknesses or resistances or not, barring influences like their size and the presence of shells.

Insectoid creatures definitely feel as though they would be weak to bludgeoning but resistant to piercing, what with their hard exoskeletons. Softer ones like caterpillars and butterflies might differ, though.

Constructs like golems and similar creatures like mimics seem like they would be weak to bludgeoning but resistant to piercing, like mundane furniture and such tends to be.

Undead might be all over the place. Skeletal creatures would definitely be weak to bludgeoning and resistant to piercing and maybe slashing. Beyond that, I'm not entirely sure. I vaguely recall hearing something about D&D having lots of undead-smiting bludgeoning weapons, but I'm not entirely sure why all undead would have a blanket weakness to bludgeoning. Vampires seem like they ought to be weak to piercing, at least, given their folklore, and maybe "soft" undead like zombies would be weak to slashing. I'm not sure beyond this, though.

I haven't the slightest idea what elementals, fey, and outsiders would be weak to. Maybe it would just vary by their size and form. My instinct screams "weak to piercing except for things like earth elementals" for some reason, but I haven't the slightest idea why.

Dragons feel as though they ought to be weak to either slashing and/or piercing, given that swords and spears seem to be popular dragon-busting weapons in stories. Bludgeoning feels right for a resistance, maybe because I don't hear about many heroes whacking dragons over the head or punching them in the face, but maybe it's just me.

Do you have any thoughts or ideas related to this? Thank you for your help!

[Jay R]

We use a fly swatter to bludgeon a fly, because its size makes it hard to slash or pierce. Shotguns are much more effective against snakes than pistols, because it's so skinny it's hard to aim. But an axe also works well against a snake.

So I would make small creatures hard to slash or pierce, and snakes or other skinny creatures hard to pierce.

I might also consider making a hard exoskeleton give a high AC for piercing or slashing, and a lesser AC for bludgeoning, but make all the bludgeoning damage non-lethal.

Which leads to the next consideration. Anybody can come up with a good idea like this that would make D&D combat slightly more realistic, and slightly more complicated. Then somebody else can do the same thing again.

Where do you stop?

[NRSASD]

Think about what each kind of damage is trying to do. A slashing wound is good for causing death by blood loss or severing. A bludgeoning wound is designed to shatter bones or armor. A piercing wound ruptures organs and exploits weak points.

[ExLibrisMortis]

Slashing and cutting is less effective against heavily armoured targets, but it's great versus anything that bleeds, and most things do die to 'separate A and B, repeat until satisfied' (however, cutting enemies into pieces is going to take some serious strength (or luck), so it's not very practical). Bludgeoning may be the easiest thing to become proficient at, maces being fairly straightforward and all. Piercing is hard to stop, but it's much less useful if your target doesn't bleed or have organs (undead, oozes, that line of work), and your weapon may get stuck, particularly for long-tipped spears, or break, as with lances.