The Weapon Proficiency feat should work like the Weapon Familiarity (Ancestry) feats for Exotic Weapons

[DoggieBert]

Assertion: As written the Weapon Proficiency feat is useless for martial characters.

Assumptions: We are either a fighter or a different martial class with the Weapon Proficiency feat to become trained in an exotic weapon. I’m ignoring the fact that all exotic weapons are uncommon and assuming you can gain access to them somehow.

Proof: For non-fighters, when we initially take the Weapon Proficiency feat to become trained in our exotic weapon of choice everything is A-ok. However, as soon as we gain expert proficiency in martial weapons from our class we don’t gain it with our exotic weapon so we are always -1 or more to hit with it. For fighters our exotic weapon proficiency always lags behind martial by 1 step, so in either case the exotic weapon has to give us a benefit that is worth at least a +1 to hit to be worth using over a martial weapon.

Exotic weapons:

• Dwarven Waraxe - This is a bastard sword with sweep. Sweep gives a conditional +1 to hit, so is always worse than a constant +1 to hit.
  
• Gnome Flickmace - As a 1-handed reach weapon this is fairly unique. I guess an argument can be made for this one
  
• Orc Necksplitter - This is a longsword with forceful. Forceful gives you +1-6 damage on your second attack and +2-12 damage on your third+ attacks. This is a decent damage increase, but unlikely to be better on average than a +1 to hit.
  
• Sawtooth Sabre - This is a shortsword with twin. Like forceful this gives extra damage on the second or later attack, but only when using a pair of them. Again, it’s unlikely to be better than a constant +1 to hit. As a side note, I don’t know how you can actually gain access to this weapon or other uncommon weapons that don’t have an ancestry trait or monk trait.

Conclusion: The Weapon Proficiency feat should allow you to treat 1 exotic weapon as martial, similar to how the Weapon Familiarity (Ancestry) feats work, rather than making you trained in an exotic weapon. This allows the exotic weapon to scale with your class level, so that it’s not a downgrade compared to martial weapons.
If we still want fighters to use exotic weapons we could give them the new Weapon Proficiency feat for free at first level so they still get access to one exotic weapon.
This allows a character to choose to invest in an exotic weapon that is slightly better than a martial equivalent at the cost of a feat, rather than spending a feat to use a weapon that is typically worse for them than the martial equivalent.

[RazarTuk]

I go into more detail here, but basically, I'm opposed to the current wording of both Weapon Proficiency and Weapon Familiarity.

Problem 1- Simple+ Proficiency

I'm going to make the assertion that if you have a character with no weapon proficiencies and an infinite number of feats, ideally, you should be able to recreate the proficiencies of any class. In 1e, the only class to fail this test is the wizard, which isn't even proficient with all simple weapons, while Simple Weapon Proficiency (a feat that's only even useful for the wizard, because even racial HD give you simple weapons) gives you proficiency with all simple weapons. But Martial and Exotic Weapon Proficiency both buy individual weapons. Meanwhile, half the classes in 2e fail, because Weapon Proficiency gives you all martial weapons.

As an example of what this implies, if I want my sorcerer to be able to swash about with a rapier and pretend to be a bard, he'd have to learn to use all weapons, not just the handful the bard knows.

Problem 2- The Tien Black Market

Ideally, weapons being common or uncommon is set for the entire setting, not an individual character. For example, Avistani longswords would be common in Absalom, but uncommon in Minkai. Meanwhile, Tien katanas would be common Minkai, but uncommon in Absalom. If I make a Tien character and the GM lets me treat katanas as common, how does that fit in with Absalom? Is there some secret Tien black market that only I can access? Or similarly, an elf can treat elven curve blades as common, so is there an elven black market in the Five Kings Mountains?

Problem 3- Exotic vs Uncommon

Exotic is redundant at the moment. Case in point, you listed literally the only four exotic weapons left, all of which could be make uncommon martial instead, with little effect on balance. The only weapons I can think of that actually deserve to be exotic are things like Numeria's lasers or Alkenstar's way-post-Medieval guns.

We've already added Uncommon Martial for "exotic meaning it doesn't fit in a pseudo-Medieval-European setting" and "exotic meaning this is too powerful compared to other weapons". So why are those four weapons special that they still deserve to be exotic?

Problem 4- Gated Guns

This one continues off the previous problem, so I'm going to make the assertion that only guns and Numerian future tech deserve to be exotic. Currently, proficiency in them is gated behind all martial weapons. In 1e, you could always take EWP as long as you had at least +1 BAB. As an example, if I were playing a rogue in an Iron Gods campaign, at any level (including even ones because of the rogue talent that gives you a combat feat), I could pick up EWP (laser gun) and learn to use these strange alien weapons we're finding. But in 2e, I would first have to learn to use all martial weapons, because Weapon Proficiency only lets you learn to use an exotic weapon if you're already trained in all martial weapons.

Conclusion

We already have well-defined weapon types for critical specialization. Let everyone be trained in all simple weapons, and instead of Martial and Simple+ as other proficiency "types", let everyone pick some number of weapon types to be trained in. The Weapon Proficiency feat would then let you become trained in another class (or possibly even an expert), critical effects could be unlocked automatically at expert proficiency, and the fighter class abilities that increase your proficiency in weapons would let you pick a handful of classes to increase your proficiency in by one step each.