Does this change to the Two-Weapon Fighting bridge the gap with GWF?

Question

Off-hand attack Rule: Replaces Two-Weapon fighting rule

When making an attack with a light melee weapon using your strength modifier for attack and damage rolls, you may make an "off-hand attack" with another light melee weapon. Your off-hand attack must use your dexterity modifier for the attack rolls and gets no modifier added to its damage. Your off-hand attack counts as being part of your main hand attack for effects that proc on attacks. If either weapon has the thrown property it may be thrown as part of this attack.

I'm also tempted to allow fists to be counted as light melee weapons, but I'm unsure how powerful that would be for monks.

My TWF Style Homebrew:

You may add half of your dexterity modifier to your off-hand attack damage.

TWF is now called "Off-hand Attack." It no longer costs a bonus action and it happens every time you attack, but now it is MAD (Multi-Attribute Dependent) and the TWF style that the fighter and ranger have applies half ability mod (rounded down).

I'm curious how this would compare to GWF. I've used a calculator that I found online from ThinkDM to try and model this, but I'm unsure if I'm using it correctly. So I'd like an extra set of eyes to look over the change and tell me what you think.

I am also curious as to what people think about whether the MAD requirement is a bit heavy for the half-ability mod. According to the calculator I've been using the half ability mod requirement brings the TWF almost directly in line with GWF, but the GWF character would get the benefits of adding ASI to constitution if they wanted to. Whereas the TWF is locked into strength and dex. Now forcing Str/Dex on a few martial characters isn't a terrible thing, but maybe I should allow full ability mod, giving the TWF more damage for shoehorning their build into str/dex. What do you think?

Here's the calculator I've been using (Google docs).

With the changes above I'm also unsure about what to do with the duel wielder feat. I think it would be cool to remove the line about how the off-hand attack counts as being part of the main hand attack. That would really allow dual wielders to shine as specialists who take great advantage of "proc on attack" spells. I'm nervous that would sersiously re-rail the power level of the feat though. I could also just leave the dual wielder feat as is, but allowing non-light weapons also sounds super powerful. Maybe if I limited the non-light weapons to the "main-hand" it would be well balanced. Thank you for your thoughts on this subject, I know it comes up a ton.

Answer 1

This does eventually overtake GWF, but it's a lot more MAD along the way

I think that calculator hides a lot of the relevant concerns here. I think we'll have an easier time looking at things if we look at a constant hit chance (we'll assume a representative 65%; difference aren't gonna warp things that much, it mostly only matters for GWM anyway) to reduce our data by a dimension, and we look at the average damage per normal attack at the fighter ASI levels.

Using the standard array and a +2, +1 race option it's possible to start with +3 in each of Strength and Dexterity. We'll be using that, maxing Strength first, and taking the relevant feat (Great Weapon Master or Dual Wielder) at ASI 3 (level 8) before maxing Dexterity. I'm ignoring the BA attack from GWM. For the sake of comparison I'm throwing in the normal Two-Weapon fighting with a per attack adjustment on the BA attack.

In general we can get the damage of an attack, DPA, as $$ \text{DPA} = h\times(D+M)+cD $$ where h is the hit chance, D is the die damage, M is the modifier (or static) damage, and c is the crit chance.

This works fine for great weapon fighting. However, since the proposed rule (abbreviating at OHA) is a bit more complicated, I'll be calculating it as $$ \text{DPA}_\text{OHA} = h\times(D+\text{STR}) + \left(h+\frac{DEX-STR}{20}\right)\times\left(D+\left\lfloor\frac{\text{DEX}}{2}\right\rfloor\right) + 2cD $$

Level	ASI	Str	Dex	GWF	OHA	TWF
1	0	3	3	7.781	7.5	8.8
4	1	4	3	8.431	7.925	10.1
6	2	5	3	9.081	8.35	8.55
8	3	5	3	9.7485	9.65	9.6
12	4	5	4	9.7485	10.525	8.53
14	5	5	5	9.7485	10.85	8.53

Now, there's a couple of things to note here. Firstly, OHA is capable of overtaking GWF, but it's investing a lot more. It forces our second highest ability to be Dex, which limits our Con to +1 (you can get that higher with point buy, by dumping your other stats hard). And until level 12, you're putting all that in for a damage output that's trailing behind GWF. If you don't invest in Dex, the damage output from OHA scales from 6.33 to 8.43 which is worse than TWF across the board. Also it's worth noting that GWM's damage increases very quickly against lower AC enemies or with advantage, so depending on factors in-play, the damage might still be worse.

A number of fighter subclasses (and the entirety of the ranger class) use other abilities (eg. Intelligence for Psi Warrior, and Eldritch Knight, Wisdom for Samurai and Ranger) which becomes very strained with this style, wanting a high Strength, Dexterity, Constitution, and the other ability. Two-Weapon Eldritch Knight becomes even worse under this variant.

It also bears mentioning that the base Two-Weapon fighting (assuming your BA is otherwise free which is fairly typical for fighters) is a fair step better than OHA or GWF during T1, and about par during T2. It doesn't really fall off until Extra Attack (2) kicks in at 11th level.

So to answer the question directly, this does appear to close the damage gap at higher levels, but at the cost of an increased rule complexity, and much higher ability investment. And while perhaps less important, OHA still has worse magic item scaling and free-hand economy. Whether the above is worth it will be up to the individual player.

While individual is of course allowed to differ, my suspicion and expectation is that most players who would be put off by two-weapon fighting because of the damage fall-off at higher levels, are gonna have similar issues with the ability cost of this option. At pretty much par damage this giving up a fair whack of hit points, and is having a harder time picking up relevant 'power' feats like Heavy Armor Master, Sentinel, Crusher, or Slasher, or taking a more evocative ability or feat (to make their character stats less just person-with-weapon), be that Inspiring Leader, Mounted Combatant, Chef, a racial feat, or something else.

So maybe closing the damage gap, but creating an ASI gap, which may or may not feel worse.