. Then you are flat out ignoring it.
No you are attmpting to declare a specific implementation as the only true way despite others pointing out that such an implementation is nearly impossible to balance around without changing way to many things already in the game.
If I were building 5e from the ground up I like your suggestion. I’m not. The implementation of a TWF fix should never require changing specific abilities to work or not work with it.
Why is it a problem to commit to the bonus action earlier? Why can’t the TWF have the choice of doing less damage now for more damage later so long as it’s sufficiently more damage later to make up the gap between a GWF using the same bonus action ability?
The big issue, for me, is that TWFing falls behind Duelist and Duelist has a shield. Watch (using base 65% to hit, assuming short rest abilities are split between two encounters).
Fighter
Level - Duelist vs TWF - TWF difference
1st - 6.4 vs 8.8 - 138%
2nd - 9.6 vs 11.0 - 115% (action surge gained)
4th - 10.6 vs 12.6 - 119% (str/dex increased)
5th - 21.2 vs 20.2 - 96% (extra attack)
8th - 23.1 vs 22.8 - 99% (str/dex increased)
11th - 34.7 vs 31.4 - 90% (extra attack)
17th - 46.2 vs 39.9 - 86% (action surge gained)
20th - 61.6 vs 51.3 - 83% (extra attack)
But, we all agree there's a problem with the fighter's TWFing, so maybe it can be fixed in the fighter?
Well, let's look at the ranger. I'm using colossus slayer, assuming the target is always already damaged (to assume otherwise would be difficult), and I'll be assuming 3 rounds of combat and that hunter's mark needs to be cast at the start of each fight (and once at the end if it doesn't need to be cast at the start of each fight as a bonus action to test how it would change, especially since Hunter's Mark starts lasting longer if you use a higher level slot). I stop at 8th level here because the 11th level hunter abilities do not favor TWFing.
Ranger w/o hunter's mark
Level - duelist vs TWFing - difference
1st - 6.4 vs 8.8 - 138%
3rd - 9.6 vs 13.1 - 137% (colossus slayer; interestingly, this is the same gain as action surge for the duelist fighter, but an even gain for the twfing ranger)
4th - 10.2 vs 14.4 - 141% (str/dex increase)
5th - 18.4 vs 19.8 - 108% (extra attack)
8th - 19.7 vs 21.7 - 111% (str/dex increase)
So, the ranger suffers the same fall off, just not as severe as the fighter. They go from dealing a fair bit more damage, 40% more, to dealing only 10% more (rounded). And remember, the duelist gets a shield.
What happens when hunter's mark comes in?
Ranger w/hunter's mark
Level - Duelist vs TWFing - Difference
1st - 6.4 vs 8.8 - 138%
2nd - 8.9 vs 12.9 - 146% (hunter's mark; remember, though, they can only do this 2 encounters a day at this point)
3rd - 12.0 vs 17.1 - 143% (colossus slayer)
4th - 12.7 vs 18.4 - 146% (str/dex increase)
5th - 23.3 vs 26.3 - 113% (extra attack)
8th - 24.6 vs 28.3 - 115% (str/dex increase)
So the ranger STILL has a considerable drop off at extra attack because of the way TWFing works. It's only marginally better than they were without hunter's mark (the percent comparison that is).
Ranger w/hunter's mark, no TWFing bonus action
Level - Duelist vs TWFing - Difference
1st - 6.4 vs 8.8 - 138%
2nd - 8.9 vs 13.7 - 155% (hunter's mark; remember, though, they can only do this 2 encounters a day at this point)
3rd - 12.0 vs 18.0 - 150% (colossus slayer)
4th - 12.7 vs 19.3 - 152% (str/dex increase)
5th - 23.3 vs 27.1 - 117% (extra attack)
8th - 24.6 vs 29.1 - 118% (str/dex increase)
Even without requiring the bonus action, there is STILL a drop off, and they end only with a 3% gain over having to use that bonus action. At higher levels, this is the reality for the Ranger if they can hold onto their concentration (poor Ranger doesn't get Con saves), or if they can just cast it before a fight starts.
For the ranger, freeing up that bonus action for casting hunter's mark in the first round of combat only amounts to a +6% damage boost at best (2nd level) and a 3% damage boost at 5th level and higher.
So it's probably not a big deal.
The rogue is where the issue is. TWFing is a 35% increase in damage. A ranged rogue can get a similar increase if they're using their cunning action to hide AND they're succeeding on their stealth checks, so I don't really want to hurt them.
If we go with my original suggestion (no bonus action requirement, TWFing Style allows use of non-light weapons), then the ranger looks like this:
Ranger w/hunter's mark, my TWFing Style
Level - Duelist vs TWFing - Difference
1st - 6.4 vs 8.3 - 129%
2nd - 8.9 vs 13.2 - 149% (hunter's mark; remember, though, they can only do this 2 encounters a day at this point)
3rd - 12.0 vs 17.4 - 145% (colossus slayer)
4th - 12.7 vs 18.1 - 143% (str/dex increase)
5th - 23.3 vs 32.2 - 139% (extra attack)
8th - 24.6 vs 33.5 - 137% (str/dex increase)
Look how much flatter that is? Rounded to the nearest 10%, that's +30, +50, +45, and then +40 from 4th level onward.
How much of a buff is this over the default ranger (if they have to use their bonus action at the start of each combat to put on hunter's mark)?
1st: 6% penalty
2nd: 2% more
3rd: 2% more
4th: 2% less
5th: 22% more
8th: 19% more
So, yes, this would be a buff to the Ranger's damage output at 5th level and on. But, I think I've conclusively proven that the TWFing warriors with extra attack lose a lot (or all of) the difference they had between TWFing and Duelist (and remember, Duelist is getting a shield).
I think I've convinced myself. Now I just need to figure out what needs to be adjusted for the Barbarian, Monk, and Rogue.
Thanks for coming to my TED-talk.