All that goes out the window once you get Extra Attack. Which accounts for 80% of a fighting class' level progression.
Greatsword becomes 4d6 + 2xstat. Which is more than 2d8 + 1d6 + 2xstat.
Agreed, but by that time all classes with extra attack as a class feature (or thirsting blade) have [+ damage] riders on attacks (Smite, Rage, Sup dice, Hunters mark, Hex, Eldritch smite, Divine Favor) or like having multiple attacks to land sneak attack in the case of rogues (who dont get extra attack), which picks up the slack.
So, bumping the main hand to 1d8 wouldn't throw balance off at all, really. It'd only make dual-wielding slightly stronger (as in, 1 measly point) from Lv. 1-4, which, again, is only 20% of character progression as well as the levels that go by quickest.
It doesnt throw balance off, but it makes TWF mechanically superior to Great Weapon fighting (and that's before you factor in Hunters mark and similar riders) at least until level 5.
It gets worse when fighting style investments are added in. A 1st level TWF fighter deals 1d8+Stat and 1d6+Stat compared to a GWF guy dealing 2d6+Stat (re-roll 1 and 2).
The TWF guy deals more damage, and has the better DPR spread (dealing damage more often).
Tagging an enemy with Hex or Hunter's Mark uses up the same bonus action you'd need to dual-wield in the first place. Making for at least one round in the battle you aren't getting the off-hand attack.
Made up for in rounds 2+.
The DPR drop off for shifting Hunter Mark/ Hex (lose 1 attack that round) more than makes up for itself from the 3rd round onwards.
Remember - in 5E monsters are big bags o HP. Unless you're dealing with a Horde of Mooks, it's always better shifting Hex/ Hunters mark on the Prime target than not.
Conclusion: Dual-wielding is NOT superior. Not even if you bumped the main hand to 1d8.
Your conclusion is flawed.
In your rules, Dual Wielding
is superior to every other fighting style (till level 5) by every measure, then from 5th level onwards, it's only superior in certain situations (Nova strikes spamming smites or sup dice, when a target is already tagged with Hex/ Hunters mark, when hit chance is more important than outright damage etc).
A more niche solution (and Im aware it's a feat tax) is to simply include a 'Superior TWF' feat (requires the extra attack class feature, dual weilder feat and TWF style).
Benefit:
- When you fight with two weapons, you may make an extra melee weapon attack with a weapon you hold in the off hand as art of the same bonus action.
- When you make an attack of opportunity and you wield a melee weapon in each hand, you may attack the same target once with each weapon you hold as part of the same reaction.
- You gain +1 to Strength or Dex to a max score of 20