Context

For a new campaign I'm building a protector aasimar kensei. We're using the Point Buy system, so being an unarmored monk I managed to raise my default AC to 15 (17 with Agile Parry), at level 3 through:

STR 10

DEX 15

CON 14

INT 8

WIS 16

CHA 10

At level 4 I was thinking of taking the Weapon Master feat, because I read online that it increases my Dexterity by 1, and lets me gain a fighting style from the fighter class. So, by picking the Mariner fighting style at level 4, I could raise my AC by 2 in just one level-up! This fighting style, from Unearthed Arcana: Waterborne Adventures, grants:

As long as you are not wearing heavy armor or using a shield, you have a swimming speed and a climbing speed equal to your normal speed, and you gain a +1 bonus to AC.

(Yes, the DM allows UA playtest material)

Now, unfortunately(!), we just discovered that according to the books Weapon Master actually doesn't let you pick a fighting style... The Player's Handbook states on page 170 about Weapon Master:

You have practiced extensively with a variety of weapons, gaining the following benefits:

• Increase your Strength or Dexterity score by 1, to a maximum of 20.




• You gain proficiency with four weapons of your choice. Each one must be a simple or martial weapon.

Discussion

Both the DM and myself agree that, by RAW, Weapon Master is rather underwhelming as a feat. So intuitively, the unofficial addition of the fighting style doesn't seem to be game-breaking to us. If anything, it makes the Weapon Master feat actually worthwhile in specific cases (instead of almost never really being worth taking it). I would say the improved Weapon Master would be comparable to the Magic Initiate feat, because by RAW the latter offers options to "obtain useful class features without multiclassing".

We think that taking a feat should be an investment worthy of not increasing ability scores with an additional point (especially when using the Point Buy system). However, we are unsure about how it could unbalance game play in ways we don't foresee right now.

Question

How would it unbalance game play to rule that Weapon Master allows for picking a fighting style, instead of four weapons?

To be clear, what does the DM need to keep in mind regarding encounter balance and player character options, when he allows the following emphasised addition to the feat:

You gain proficiency with four weapons of your choice. Each one must be a simple or martial weapon. Instead of gaining proficiency with four weapons, you may gain a fighting style of your choice from the fighter class.

A good answer addresses mechanical implications that makes this house-rule balanced or imbalanced: in comparison to how Weapon Master functions by RAW, and compared to other feats. A good answer addresses this in the context of our specific table.

An excellent answer also shares experience from a table that allows this house-rule, covers potential exploits and how it effects their game mechanically.

Possibly relevant details

• I won't be the only melee combatant in this party, composition so far: barbarian, monk (me), war cleric, wizard.




• We start at level 3. If our characters survive, we'll get to level 5 in just a few sessions (through milestones). After level 6 I'm seriously considering to multiclass into fighter, rogue or cleric. I plan to never wear any armor or a shield, so just wield the kensei weapons.



• The DM wants this setting to be leaning towards a "Ragtag band of adventurers". He also prefers to keep mechanics balanced at the table, both for the players and him as DM. The campaign will be heavy on combat: we are holy crusaders.

It wouldn't be unbalancing provided it can't be stacked, but it is on the strong end for feats

I handled this by mechanically evaluating it as well as looking for appropriate parallels.

At level 3, your AC is usually 17: 10 + 2 (Dex) + 3 (Wis) + 2 (Kensei feature)

With the proposed rule your AC would increase as follows:

At level 4, your AC is 19 most of the time: 10 + 3 (Dex) + 3 (Wis) +2 (Kensei feature) + 1 (Fighting Style)

By levels 8, 12, 16, and 19 your AC could be 20, 21, 22, 23, respectively (by putting points into Dex and Wis).

By bounded accuracy, this isn't unreasonable as a Fighter could easily attain the same AC by making a comparable effort to increase AC.

Compare this to the alternative of not permitting the feat change.

At level 4, your AC is 18 most of the time (regardless of whether you take an ASI for Dex or a feat that gives 1 point of Dex).

By levels 8, 12, 16, and 19 your AC could be 19, 20, 21, 22, respectively.

In the proposed scenario, the character has given up 1 point of ASI to increase their AC by 1 point. I think the biggest issue, though, is that there isn't really precedent for this. There are feats that give a 1 point ASI plus another feature (Athlete, Actor, Durable, various armor feats, Keen Mind, etc.) There's also one that gives extra AC, with conditions (Dual Wielder). The problem is, there aren't any that do both directly.

That said, there are some indirect feat comparisons that I think are worth consideration. The Resilient feat permits you to increase an ability score and gives proficiency in the save, it's a very popular choice for a player to pick to provide a substantial increase to defenses. Another choice is Heavy Armor Master, which provides straight damage reduction for each hit taken from non-magical damage, which is actually quite a lot of the MM.

There are also feats that provide access to class features in the form of Martial Adept, which gives you two Battlemaster maneuvers and superiority dice for it. This is a means for a character to get access to a powerful 3rd level class feature via a feat.

Another indirect comparison is to look at how Fighting Styles are provided. The Fighter, Paladin, and Ranger all receive Fighting Styles as a class feature. The Fighter does not gain their Fighting Style by itself, at 1st level they gain it with Second Wind; the Paladin does not gain their Fighting Style by itself, at 2nd level they gain it with spellcasting and divine smite; and Rangers also get spellcasting with their Fighting Style at 2nd level. It should be noted that barring exceptional circumstances, none of those classes get Fighting Styles more than once and in no circumstances are they stackable. So there's a sort of implied consensus that Fighting Styles are not exceedingly powerful to be the sole class feature gained on a level, but they also should not be stacked.

Essentially the querent is asking to lose out on 1 point of ASI in exchange for a 1 point increase in AC. It would need the standard caveat for Fighting Styles that repeats are not permitted and also need to exclude language that would permit the character to take the feat more than once.

In summary, I think the proposed feat isn't gamebreakingly unbalanced, but I think it is a very strong feat that would warrant moving it way up the tiers of feats. My primary reason for thinking this is OK stems from the Resilient feat giving proficiency in a save (such as Wisdom); that Fighting Styles are never given by themselves; and that other feats granting access to class features do so for features that occur at 3rd level, not 1st or 2nd.

Fighting Styles are powerful. If you wanted a feat which granted a Fighting Style, I would make it a feat which does just that and does not also boost Dexterity.

A casual glance through the 5e feats reveals two tiers of feats. There is the 'upper' tier of feats, which provide some powerful benefit which can be worth trading two points of Ability Score Improvement for. Then there is the 'lower' tier of feats, which provide some lesser bonus along with a single point of Ability Score Improvement.

The Weapon Master feat, as written, is one of these 'lesser' feats as you have noticed, because gaining weapon proficiencies is generally underwhelming. It might increase your weapon damage by one or two, but most classes which rely on weapons for damage already have adequate weapon proficiencies, so it is generally not needed unless you have some specific build which requires weapons your class does not have.

However, a Fighting Style is a very potent feature. Fighting Styles can do things such as granting +2 to attack, +2 to damage, +1 to AC, adding your ability modifier to off-hand damage (which could grant up to +5 damage per round), or imposing disadvantage on attacks as a reaction (each requiring the use of particular weapons or equipment). These benefits would lie in the upper tier of feats.

Let us analyse to some upper tier feats (as in, feats which don't also grant an ability score improvement) to see if a Fighting Style makes for a balanced feat. Charger gives a situational +5 to damage. Dual Wielder gives +1 to AC and maybe a couple extra points of damage. Medium Armour Master can give you +1 AC (and can make you sneakier). Polearm Master gives you a bit of extra damage and an extra way to make opportunity attacks. Savage Attacker gives you some extra damage. These feats provide benefits similar to a Fighting Style in magnitude (perhaps even slightly greater in certain circumstances). This would suggest that granting a Fighting Style as a feat is probably balanced.

For comparison, none of the lesser feats (as in, ones which do grant an ability score improvement) in the Player's Handbook provide any comparable combat bonuses. The most combat-related lesser feats are Weapon Master and Heavy Armour Master (which reduces some damage by 3). This means that a Fighting Style is not suitable as a lesser feat, one bundled with a one-point Ability Score Improvement.

I disclaim that I have not attempted to grant Fighting Styles via feats in any campaign I have run or played in. But I recommend that, if you wish to do this, you do not modify the Weapon Master feat but instead create a new feat:

Fighting Style

Gain one Fighting Style of your choice from the fighter class. You can't take a Fighting Style option more than once from any source.

You can select this feat multiple times. Each time you do so, you must choose a different Fighting Style.

Caveat: this is untested. It is possible that granting Fighting Styles to classes which are not meant to have Fighting Styles might cause unforeseen balance issues, although since feats are of a similar power I find this unlikely. One might also consider such a feat as stepping on the toes of martial classes such as fighters, paladins and rangers, although since you have none of those in your party this is probably not an issue for you. But if you are wanting to homebrew in a way to pick Fighting Styles with feats, this is probably a good place to start.

This "Feat" is a dangerous interaction with Monk

It's likely too strong overall, and definitely too strong in your case.

The thing about that Fighting Style is that it's not that strong for Fighters. If it were a fighter wanting to take a feat for it, I'd probably allow it as a one-time case-by-case dispensation. You can't use a shield or heavy armor, so unless you take another feat or magic items, you're bound to 18 AC.

Increasing an ability modifier is fine. Increasing your AC and some mild utility (speeds) is fine. Doing both, without any opportunity cost, is troublesome. Taking a Fighter dip delays your Monk progression. None of the feats that increase AC also increase a stat (such as Medium Armor Master or Dual Wielder) and wouldn't be super useful for a Monk anyway. This feat removes opportunity cost.

Monks get a bonus to their Armor Class from their Wisdom, so you'll already be at a 15 AC... taking your proposed +1Dex and Mariner at level 4 bumps you up to 17 AC. Then every Ability Score Increase (ASI) an increase it further, while also increasing your "Monkiness" (not to be confused with monkeyness) and/or damage.

This problem is not abated by any of your Multiclass options as they share the requisite abilities and give you further ways to keep yourself out of trouble (Healing Surge, Spellcasting, Cunning Actions, etc). You could very easily and realistically wind up with 21 AC, with no magic items needed.

It basically becomes a feat tax for Monks. It offers a benefit every Monk would want with no downside and that makes it too strong.

Pyrotechnical points out that you could Multiclass to gain this Fighting Style. This is true, you wouldn't also gain the +1Dex, but you would gain some other things, like proficiencies that you wouldn't use. It brings up a good question:

Why not just wait until you multiclass at level 5 (or whatever) and choose the Mariner Fighting Style?