That was probably me.
I was also considering removing GWM and SS -5/+10 feature, making the extra attack a crit/kill of GWM baseline of combat, as the -5 becomes trivial late while the +10 is too good to pass, making it obligatory to use it if you want optimized combat.
I also thought on making other features from feats baseline, like Charger, Defensive Duelist, PAM's Attack of Opportunity, Mounted Combat, Shield Master's shove.
But talking about TWF, I'd be ok with the offhand attack being free. The damage output of 2 scimitars would be higher than a greatsword.
2 Scimitars (1d6 + PB + Ability Score + 1d6 + PB) vs Greatsword (2d6 + PB + Ability Score)
The trade-off here is that you'd need to hit both attacks and crits with the greataxe/greatsword are better.
With this, you make technically balance TWF vs 2HF, as it doesn't force the expenditure of the bonus action.
Until extra attack comes, which opens the discussion of the extra attacking with the offhand.
Granting extra attack (i.e. 4 attacks at 5th level) damage-wise doesn't seem that much, the only problem someone pointed out is that Versatile weapons become weak with the change, so you'd need to buff their Versatile damage or something.
The problem with this is that the bonus action competes with Bardic Inspiration and Rage/Frenzy, not to mention Rogue's Cunning Action, it's too high a cost versus just equipping a greatsword and not having to worry about the trade.
Offhand attack as BA still makes it bad on every class but Fighter.
Offhand as free action:
1 Rogue, 1 Attack, 1 Offhand (+ Sneak Attack, + BA)
1 Barbarian, 1 Attack, 1 Offhand (+ BA)
5 Barbarian, 2 Attacks, 2 Offhand (+ BA)
3 Berserker Barbarian, 2 Attacks, 1 Offhand
5 Berserker Barbarian, 3 Attacks, 3 Offhand
1 Fighter, 1 Attack, 1 Offhand (+ BA)
5 Fighter, 2 Attacks, 1 Offhand (+ BA)
11 Fighter, 3 Attacks, 3 Offhand (+ BA)
20 Fighter, 4 Attacks, 4 Offhand (+ BA)