Bonus Action Limitations of Two Weapon Fighting

Raging is a bonus action. If you want to rage, you'll be unable use two weapon fighting that turn. This can put you at a disadvantage right from the start of combat.

Great Weapon Master Bonus Actions

It's not just a critical attack that allows a great weapon master to attack with their bonus action.

On your turn, when you score a critical hit with a melee weapon or reduce a creature to 0 hit points with one, you can make one melee weapon attack as a bonus action.

This is still conditional, and will depend on the kind of opponents the DM throws at the party, but mobs of foes and minion types often make appearances in many campaigns and will greatly boost the damage of a great weapon master.

Growth from Extra Attack is in favor of great weapons

With a great weapon, the Extra Attack feature makes you go from 1 attack per turn to 2 (barring great weapon master conditional bonus action attacks). With a two weapon fighter, you go from 2 attacks to 3. So while the damage of the great weapon increases by 100%, the two weapon fighter only gets a bit more then a 50% damage boost (unless you have the two weapon fighting style, in which case, it is a 50% increase).

This same issue also means a great weapon barbarian benefits more from the single extra attack granted by the spell Haste. It also pops up if you decide to multiclass into fighter, because action surge doesn't grant you another bonus action.

Problems created from using more than one weapon

There are many spells you could potentially have applied to your weapon by a party member, such as Magic Weapon

2nd Level Transmutation Spell

You touch a nonmagical weapon. Until the spell ends, that weapon becomes a magic weapon with a +1 bonus to attack rolls and damage rolls.

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the bonus increases to +2. When you use a spell slot of 6th level or higher, the bonus increases to +3.

However, this spell and many others only work on a single weapon at a time. So while a great weapon user has all their damage become magical (allowing it to bypass resistance to non-magical damage, which is critical against many foes), a two weapon fighter is left with only part of their attacks improved.

Consider your role in the party

Reckless attack isn't just a risky move, but a way to control opponents. Granting your opponents advantage encourages them to attack you, rather than your squishy wizard friend. With rage, and especially bear totem rage, you can take the damage. If your build is too defensive and never uses reckless attack, opponents may ignore you and go after bigger threats rather then one they can barely hit and can't deal full damage to.

Advantage has a huge effect

This is kind of obvious, but maybe it'll help illustrate just how useful reckless attack is for someone with great weapon master.

Source

If you look at this chart, you can see you have an 75.1% chance of rolling a 11 or higher on your roll if you have advantage. With 18 strength and a +2 proficiency (what you'd probably have at level 4), that's enough to hit 12 AC target, which is the AC of plenty of big beasts and smaller enemies. You'll still hit 15 AC targets 57.8% of the time (without advantage and without the -5 to hit, you'd normally hit 60% of the time with the same stats).

And against very high AC targets, you can choose to not use the power attack and still be relatively effective.

This change is unbalancing as it makes the Dueling Fighting Style often strictly better than the Great Weapon Fighting Style

Let's look at the Dueling Fighting Style (D-FS), it states:

When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon.

This is quite clearly meant to benefit those who wield a single one-handed weapon. The other hand could hold a shield, some item such as alchemist's fire or, in the case of a Swords Bard, Paladin, or Ranger, even be free to use a component pouch.

Also of note is just how much this increases your damage by; increasing the damage die size increases a weapon's average damage by 1. As such, increasing the damage flatly by 2 is similar to increasing the damage die size twice. This means 1d8 versatile weapons would be similar to 1d12 weapons (an average damage of 6.5) and 1d10 versatile weapons would be similar to 1d14 weapons (an average damage of 7.5).

Let's compare this damage increase to that of the Great Weapon Fighting Fighting Style (GWF-FS) which states:

When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die and must use the new roll, even if the new roll is a 1 or a 2. The weapon must have the two-handed or versatile property for you to gain this benefit.

GWF-FS, as discussed in the question "How much damage does Great Weapon Fighting add on average?, increases your average damage by (X-2)/X where X is the size of your weapon's damage die. Note, this number is always less than one, and so it is always worse than just increasing the die size once, let alone twice.

This change to D-FS would make it strictly better than GWF-FS in all cases except the 2d6 greatsword. Additionally D-FS comes with versatility of allowing the character to take up a shield (or other non-weapon) and still gain the benefits of the Style which is not the case with GWF-FS.

Below is a chart showing these differences, note that D-FS stops increasing after 7.5 because this is the maximum average damage when using this feature (when using a 1d10 versatile weapon).

Note: The "Difference" column is D-FS's average damage minus GWM-FS's average damage, so if it is positive then D-FS is the strictly better option.

\begin{array}{|lcccc|} \hline \text{Die} & \text{Average} & \text{GWF-FS Average} & \text{D-FS Average} & \text{Difference} \\ \hline \text{d8} & 4.5 & 5.25 & 6.5 & 1.25 \\ \hline \text{d10} & 5.5 & 6.30 & 7.5 & 1.2 \\ \hline \text{d12} & 6.5 & 7.33\bar{3} & 7.5 & 0.166\bar{6} \\ \hline \text{2d6} & 7.0 & 8.33\bar{3} & 7.5 & -0.833\bar{3} \\ \hline \end{array}

As you can see D-FS is better than GWF-FS in all cases except the 2d6 greatsword. This is clearly a bad move, as it completely overshadows the other Style.

Here is a chart showing what would happen if you restricted your D-FS change only to 1d8 versatile weapons (the D-FS's average damage would now max out at 6.5, when you are using a 1d8 versatile weapon):

\begin{array}{|lcccc|} \hline \text{Die} & \text{Average} & \text{GWFS Average} & \text{D-FS Average} & \text{Difference} \\ \hline \text{d8} & 4.5 & 5.25 & 6.5 & 1.25 \\ \hline \text{d10} & 5.5 & 6.30 & 6.5 & 0.2 \\ \hline \text{d12} & 6.5 & 7.33\bar{3} & 6.5 & -0.833\bar{3} \\ \hline \text{2d6} & 7.0 & 8.33\bar{3} & 6.5 & -1.833\bar{3} \\ \hline \end{array}

This makes the Style slightly better, allowing it to apply to the Quarterstaff, Spear, and Trident while they are two-handed. D-FS already can be applied to one-handed 1d8 weapons, so applying it to a two-handed quarterstaff won't increase the power outside of a Monk multi-class; however, the spear and trident can both be thrown so the Style gains some additional power as you now have a choice between a 1d8 weapon or a thrown 1d6 weapon.

Normally a character could wield a 1d8 one-handed weapon and a 1d6 thrown weapon, and to get a free hand, they would have to either drop a weapon (which can possibly go poorly if it is kicked away or stolen) or put the weapon away (which takes an object interaction).

With this change a character could go from wielding a 1d8 D-FS versatile weapon to having a free hand and a 1d6 thrown weapon with no cost to their action economy.

This is an incredibly specific/niche use though so I don't see it having much of an effect on gameplay. (It requires a character to want both a thrown weapon and a melee weapon out, and for some reason now need a free hand, and their object interaction; this seems like a niche enough case to me).

D-FS (even unchanged) is better than GWF-FS unless a character is specifically using a 1d12 weapon or the 2d6 greatsword. The difference in the average damage between 1d8 D-FS weapons and 1d10 GWF-FS weapons is only 0.8333.

I personally do not feel that this is enough of a difference to warrant taking GWF-FS given D-FS's greater versatility in allowing you to take up a shield (or other non-weapon).

As a result the GWF-FS (unmodified) is restricted to the 2d6 greatsword, characters who will not use a shield (such as Barbarians), or classes such as Barbarian or the Champion Fighter and possibly Paladin, where critical hits are more favorable or more likely than usual (crits roll more dice, so for them, the rerolls from GWM-FS are better than the flat +2 from D-FS). This is already a thing, and the modification to your change suggested here doesn't make that gap any worse.

Making your change in general makes the Great Weapon Fighting Style strictly worse unless you are using a greatsword.

Making your change only apply to two-handed versatile 1d8 weapons is quite reasonable as the feat already applies to one-handed 1d8 weapons.