So, Fighter.
You all know the meme of 'Human Fighter'. It's as basic you can get: no worrying about being a multi eyed paranoid horse-thingy, no getting worked up setting up sneak attack, no spell slots to worry about, you just go up to the enemy and hit them in the head until they die.
Easy, clean and simple.
Now, Pathfinder Second Edition has brought a whole slew of improvement for the Martial classes, allowing them to contribute to the team as much as their spellcaster friends.
Among these changes, Fighter was given more of an identity to call its own. In D&D, Fighter is the class that... fights good. Kind of vague, don't ya think?
In Pathfinder 2e, Fighter is the class that... fights good. OK, that hasn't actually changed, but it now actually means something, thanks to the class' unique niche of being one step ahead in weapon proficiency compared to other Martials of the same level, allowing them to hit AND crit more often (Shut up Gunslinger. You too, Rogue. Off guard is not a proficiency bump.)
And that's where the problem lies.
See, most martials have a preference for weapons due to their mechanics: Barbarians don't like Agile weapons, Rogues want to use weapons with Finesse and Thaumaturge suffer by being a martial with a Key Attribute different from Strength or Dexterity etc.
Now, you'd think that Fighter with their higher proficiency would be the most free to choose a weapon, and for the first 4 levels you'd be right, but once you hit Level 5 you can say all of that goodbye, because Fighter Weapon Mastery forces you to commit to a single Weapon Group.
Want to use that cool Lance you found that shoots lighting? Too bad, you choose Polearms for your weapon group, and Lances's weapon group is Spears, not Polearms (why are they different we may never know), so you sell it because the rest of the party is playing Thaumaturge and you don't have the time to retrain.
Now you might say this is in exchange for some class feature Fighters get at level 5.
Well, let's see:
Nothing. Fighters don't get anything that other Martials don't also get at that level.
In fact, you could argue they get less, since other Martials have their weapon proficiency uniformly increase, even for weapons they cant' really use. The only other thing unique to Fighter is Combat Flexibility, and that is a Level 9 Feature.
You could instead say that's to keep the class from becoming too strong, except that's not true, since for the first 4 levels they get free reign to pick up any weapon they like (except Advanced ones but they're another can of worms I'm not gonna get into) and swing it around with their higher proficiency. Not only that, but once a Fighter reaches Level 19, they gain the Versatile Legend feature, allowing them to once again ignore weapon groups just like in the good old days. And I haven t heard a thing about Fighters terrorizing those level ranges.
So, we've established that Fighter in its current state is literally unplayable by being forced to only pick a single type of weapon.
Where do we go from here?
Well, for all I ranted about Fighter Weapon Mastery being a bad feature, the idea of a Fighter specializing into a single Weapon Group isn't a bad one.
It just needs some polish:
First of all, it needs to be a choice on whether you focus on a single weapon group, or none at all.
Second, the choice should be made at level 1. Such a core part of a character's build should be made immediately, to let the character invest in it ASAP without it being kind of a waste (I'm looking at you Invulnerable Rager and Harbinger's Protection).
Third, there needs to be some kind of actual benefit for the tradeoff. Thankfully, Paizo has already done my job with the Slinger's Precision class feature: bonus precision damage, either +2 or +1d4. Since Fighter is all about consistency, I'll go with the +2 damage.
If that's too strong, we can instead let Fighters use Advanced Weapons of their weapon group without incurring in the proficiency drawback, since the budget for them is only one step of the damage die higher compared to Martial weapons. Whatever the bonus is, proficiency for weapons of other groups should be reduced to Trained and remain a step behind. Also, remove Versatile Legend, it's obsolete either way.
And if a Fighter doesn't want to specialize, no problem. They'll still get to enjoy a higher proficiency with their weapons.
So TL;DR fighter should have a choice between specializing or not, all the while keeping their higher proficiency bonus.