So RAW, you don't add any proficiency to a untrained weapons. So a level 10 sorcerer just adds their strength to an attack roll they make with a greatclub, with very little chance of success.
BUT! Improvised weapons are treated as simple weapons that just take a -2 penalty to attack rolls. That means a sorcerer is better off swinging a chair or tree branch than something actually designed to be swung. Which, obviously, seems wrong.
Logically, a weapon you aren't proficient with should be usable as an improvised weapon, unless it is something you're more likely to stab yourself with like a Star Knife. A -2 penalty by PF2 math is also pretty close to the -4 non-proficiency penalty under PF1 math.
Am I missing some rules interaction that prevents this? And if I allow it as a house rule, am I missing any awful ramifications?
Overall you're fine.
Minor questions though:
1. How would one determine which weapons are too pokey, like a starknife, and which ones weren't?
2. How much damage is the weapon doing?
An untrained somebody swinging a greatclub as a greatclub would get greatclub damage. If they swung it like a big stick (even if the concepts are nearly synonymous) they'd likely do less damage, so there's that. If the weapon's magical, the difference would be significant.
Yet then the improvised weapon might get bonus damage from Weapon Specialization (since it counts as a simple weapon) while the real weapon doesn't (since the user has zero proficiency).
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Personally I'd go with being able to use an untrained weapon like an improvised weapon, not just because that'll typically be superior (barring extraordinary magic or circumstances), but also because it's plausible.
Of course you could swing a sword like a stick, forgetting the niceties of its tip and edge. To the untrained, they are improvising with it, using it in a way most similar to the weapons they know, not the weapon it is.
But that would reduce its damage die in most cases plus change the weapon's traits.
Overall you're fine.
Minor questions though:
1. How would one determine which weapons are too pokey, like a starknife, and which ones weren't?2. How much damage is the weapon doing?
An untrained somebody swinging a greatclub as a greatclub would get greatclub damage. If they swung it like a big stick (even if the concepts are nearly synonymous) they'd likely do less damage, so there's that. If the weapon's magical, the difference would be significant.Yet then the improvised weapon might get bonus damage from Weapon Specialization (since it counts as a simple weapon) while the real weapon doesn't (since the user has zero proficiency).
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Personally I'd go with being able to use an untrained weapon like an improvised weapon, not just because that'll typically be superior (barring extraordinary magic or circumstances), but also because it's plausible.
Of course you could swing a sword like a stick, forgetting the niceties of its tip and edge. To the untrained, they are improvising with it, using it in a way most similar to the weapons they know, not the weapon it is.
But that would reduce its damage die in most cases plus change the weapon's traits.
1. I'd just make the call on the specific weapon in question, I think. If it doesn't seem plausible, I'd leave proficiency at 0. Bladed Scarf is one of the few examples I could think of. I suppose a bow and arrow might not work, less because it is dangerous and more because I don't think proper archery has much reassembly to firing a crossbow or lobbing a tankard at someone's head. But a bomb is pretty close to lobbing a tankard, by contrast.
2. Depends, I guess. Lowering the weapon damage die a step seems like a decent baseline.