There are also the ones I mentioned in my initial post in the thread.
Ok, here is your first post, numbered for convenience:
I think it is important to consider why you are making this change.
(1)If you feel that fighting with two weapons isn't offensively powerful enough, (2)despite the added versatility of being able to use two different weapons and (3)make melee or ranged attacks in the same round, (4)as well as getting two opportunities to hit and apply special effects like sneak attack, (5)and getting two chances to score a critical hit - then this rule change might be suitable as it (6)makes fighting with two weapons more powerful.
If you are thinking of a different reason for the change, then my thoughts depend upon that reason.
1. No, I don't feel that fighting with two weapons is enough, but I assume you didn't actually intend that as a reason. So moving on!
2. I don't see a lot of benefit in being able to use two different weapons. Maybe different damage types? Like slashing and piercing? But then you are more likely for one of your weapons to be resisted, so that kinda balances out. And again, needing two weapons means needing two magic weapons instead of just one. And you are limited to only 3 attuned items. Sure it's only a problem for higher level characters, but it is still there.
3. Again, a minor boost. You can only draw one weapon during your turn. (Unless you get the Dueling Feat, but that's a Feat. If we add that we have to add the possible bonus attack from Great Weapon Master.) My greatsword Eldritch Knight Fighter commonly carries a javelin that he throws before switching to his greatsword. He then moves and attacks thanks to Extra Attack. If he needs a ranged attack again he releases the greatsword with one hand, summons the javelin back to his hand (a bonus action with Weapon Bond), throws it, then grabs his sword again. Or he could just draw a new javeling to throw and he wouldn't even need to use the bonus action. So maybe a slight advantage from levels 1 to 4?
4. Of course the Rogue already gets two opportunities to get his Sneak Attack with two-weapon fighting, and my proposed rule doesn't change that. It just means he still has his bonus action for a Dash, Disengage or whatever. That said I have since decided, mostly for this exact reason (and Martial Arts), to put the removal of the bonus action into the Two-Weapon Fighting Style. If a Rogue or Monk wants to dip Fighter for the style, and delay all of their actual class abilities by a level, so be it.
5. Two chances to get a critical hit for half the benefit is breaking even. I definitely don't see this as an advantage.
6. Slightly more powerful, yes. It doesn't actually increase the damage done by two-weapon fighting at all, it just removes the times when you would do less damage because you needed your bonus action for something else.
As there should be. Choices aren't choices if there aren't pros and cons to all of the options meant to be chosen from.
But the pros should equal the cons. I don't see that being the case. It has been shown that two-weapon fighting actually does
less damage than a sword and board Fighter and has the same AC as a two-handed Fighter. All this for the debatable advantages listed above.
A two-handed weapon has the best damage output (usually - particular two-weapon builds can deal some potent amounts of damage that while not beating out a two-hander can at least remain competitive enough), a one-handed weapon and a shield has the best defense, and the two-weapon user splits the difference with damage and defense potential of the other two and picks up some versatility that, in some people's view, makes up the difference.
What two-weapon builds? I'm being completely honest here. If there is a two-weapon build that significantly improves the damage output I haven't seen it (putting aside Rogues that get an obvious advantage of an extra chance to land a Sneak Attack). And how does the two-weapon user split the difference on damage and defense? He does
less damage and is tied for the worst defense. Are we talking about the Dual Wielder feat?
Let's talk about the Dual Wielder feat. It adds one extra point of average damage on a hit and +1 AC. Still not sure the extra damage actually surpasses the Dueling Fighter, but you can at least get close. The +1 AC is nice. Oh, and you can now be ready to fight with your preferred style in one round instead of two like everyone else. Yay?
But then the others get Feats too. The two-handed Fighter gets Great Weapon Master, which give him an extra attack if he drops a foe and the -5 attack, +10 damage that is huge against low AC monsters. Commonly considered one of the top broken Feats.
Meanwhile the sword and board Fighter gets Shield Master giving him a bonus action Shove, which can improve his damage by giving him Advantage, and an Evasion effect which increases his defense even more.
There is also a factor of dealing the "worst" damage not being much to be concerned about because you are still dealing enough damage (because if the game set the "enough" threshold at actually being optimize for dealing damage, I don't think there would be so many folks decrying the monsters in the books as "too easy.").
Certainly a two-weapon Fighter isn't a waste of space, but if you choose to go with two-weapons you probably envision yourself as focusing on offense. I know I do. So it is a little disheartening to find out that the defensive sword and board Fighter is actually
better at offense that you are, even if only slightly. And then is significantly better if you both need to use a bonus action.
I will say that I am very glad I posted this question. I loved the suggestion to move the bonus action removal to the fighting style, and further analysts is convincing me that I should probably do even more to boost the damage of two-weapon fighting to put it just a little ahead of dueling with a one-handed weapon. Not sure what to do with that yet though.
Edit: Looked at the damage again. I think I can just go with the bonus action removal and allow two-weapon fighters to get the extra attack whenever they use an Attack Action. From 1st through 4th the two-weapon user has a definite advantage. After everyone gets Extra attack they are still slightly ahead of the single weapon fighter. I do allow Feats and the Dual Wielder Feat should be enough to push them ahead when they get Extra Attack 2. So yeah. Get rid of the required bonus action with the Two-Weapon Fighting Style and things look much better.