Dual Wielding just needs to let us use second weapons special skills that is all. And Gwm needs to be nerfed in half cos we all know that feat is broken.
When you do the avg numbers on GWM it usually isn't that strong without a way to boost accuracy*, but this is where the homebrewed stuff in Larian comes in. There are much more ways to increase accuracy in BG3 compared to tabletop, which means GWM scales very high, very early.
*Don't believe me? Here is a Fighter using GWM, a Blastlock and a TWF Hunter Ranger dmg numbers at lvl 3, 4, 5 and 11.
Fighter
Lvl 3 - 2d6 + 3 = 10 x 0.65 = 6.5
Lvl 4 - 2d6 + 3 + 10 = 20 x 0.35 (- 5 from GWM, -1 for not increasing str) = 7
Lvl 5 - 4d6 + 6 + 20 = 40 x 0.35 = 14
Lvl 11 - 6d6 + 15 + 30 = 66 x 0.4 (- 5 from GWM, no penalty from str since we have bonus ASI at lvl 6) = 26,4
Blastlock (Eldritch Blast + Agonizing Blast + Hex + increase Charisma at lvl 4 and 8. Often considered the benchmark to meet for acceptable dmg per turn)
Lvl 2 - 1d10 + 3 + 1d6 = 12 x 0.65 = 7.8
Lvl 5 - 2d10 + 8 + 2d6 = 26 x 0.65 = 16.9
Lvl 11 - 3d10 + 15 + 3d6 = 42 x 0.65 = 27.3
TWF Hunter Ranger
Lvl 2 - 2d6 + 6 = 13 x 0.65 = 8.45
Lvl 3 - 2d6 + 6 = 13 x 0.65 = 8.45 + 3,9 (from Colossus Slayer with 2 chances to trigger) = 12.35
Lvl 5 - 3d6 + 12 = 22.5 x 0.65 = 14.625 + 4,3 (Colossus Slayer with 3 chances) = 18.925
Lvl 11 - 3d6 + 15 = 25.5 x 0.65 = 16.575 + 4.3 = 20.875
TWF Hunter using Hunter's Mark at lvl 5.
Round 1 (settup): 2d6 + 8 = 15 x 0.65 = 9.75 + 3.9 = 14.05
Round 2 (full potential)**: 3d6 + 12 + 3d6 = 33 x 0.65 = 21.45 + 4.3 = 25.75
Note: Ranger 5 -> Rogue multiclassing can add ~2.9 (2 attacks) to ~3.3 (3 attacks) avg dmg per turn for each Sneak Attack dice without spending any resources or feats.
**This will require no settup in OneD&D, since TWF is no longer a BA there, and Hunter's Mark won't need concentration for Rangers. Ranger go brrrr in OneD&D playtest.
Contrarily, here is a Barbarian using Rage + Reckless Attack + GWM (from lvl 4). Barbarians are great users of GWM because they get advantage for free.
Lvl 2: 2d6 + 3 + 2 = 12 x 0.8775 = 10.53
Lvl 4: 2d6 + 3 + 2 + 10 = 22 x 0.5775 = 12.705
Lvl 5: 4d6 + 6 + 4 + 20 = 44 x 0.5775 = 25.41
