Xeviat
Dungeon Mistress, she/her
Two Weapon Fighting could have been balanced without the bonus action. The only classes that would overly benefit from this are the Monk and Rogue. TWFing would become the default melee style, more so than it is now, since it wouldn't compete with the bonus action for aim or cunning action.
TWFing could have also scaled with extra attack. I've suggested this before but it usually gets overlooked:
Greatsword is 2d6+Str
Two shortswords is 1d6+Stat +1d6. There's an advantage of being able to split attacks, so that's something.
If the TWFing style didn't give stat to offhand, and, say, let you two weapon fight with nonlight weapons, then the style would be equal to the +2 damage from duelist (I'd suggest changing Great Weapon Fighting to +1d4 damage, so that it multiplies on a crit and so it's the same for 2d6s and 1d12s).
But what about magic weapons? "TWFing can stack magic weapons" you say. If you're talking about basic magic weapons, it's not a big deal and just make sure you're giving out items equally (Joe has a +1 sword and a +1 armor, Jill has a +1 sword and a +1 shield, and Jane has two +1 swords). If you're talking about special weapons, you have attunement caps and the great weapon fighter can stack a belt of giant strength and a magic greatsword so it's fine.
Rogues and monks would need to be addressed. Flurry would need to be +1 attack, martial arts would basically always be on. You'd have to accept rogues would not have to choose if they use cunning action, and you might need to do something like give the rapier no shield rogue a +2 to hit (I want that style anyway, I've done the math it's ok and cool).
TWFing could have also scaled with extra attack. I've suggested this before but it usually gets overlooked:
Greatsword is 2d6+Str
Two shortswords is 1d6+Stat +1d6. There's an advantage of being able to split attacks, so that's something.
If the TWFing style didn't give stat to offhand, and, say, let you two weapon fight with nonlight weapons, then the style would be equal to the +2 damage from duelist (I'd suggest changing Great Weapon Fighting to +1d4 damage, so that it multiplies on a crit and so it's the same for 2d6s and 1d12s).
But what about magic weapons? "TWFing can stack magic weapons" you say. If you're talking about basic magic weapons, it's not a big deal and just make sure you're giving out items equally (Joe has a +1 sword and a +1 armor, Jill has a +1 sword and a +1 shield, and Jane has two +1 swords). If you're talking about special weapons, you have attunement caps and the great weapon fighter can stack a belt of giant strength and a magic greatsword so it's fine.
Rogues and monks would need to be addressed. Flurry would need to be +1 attack, martial arts would basically always be on. You'd have to accept rogues would not have to choose if they use cunning action, and you might need to do something like give the rapier no shield rogue a +2 to hit (I want that style anyway, I've done the math it's ok and cool).