Li Shenron
Legend
Hm, so the goal here is to make the fighter a more attractive, distinctive class, with its own unique mechanical trick ("extra attacks")?
So what I'd recommend is rolling the damage from an extra attack into the other class's big mechanical tricks. So maybe the Barbarian's rage adds EVEN MORE damage, and the Rangers TWF perhaps deals MORE DAMAGE if both attacks hit.
How much damage is kind of an open question, since the maths weren't locked down when we saw the last playtest doc, but you could probably chuck a d6 or a d8 into that and be fine.
Sounds good!