Would changing Two-Weapon Fighting to this:
Off-hand weapon is light and
Two-Weapon Fighting feat +0 +0
make it more balanced against using a two-handed weapon with power attack?
TWF has problems that are multiple when compared to S&B or THF. If you want to address the problem, you have to answer a couple of questions:
1) How many feats should be required to make it equal to THF in damage?
2) How many feats should be required to make it equal to S&B at defense?
3) In doing either #1 or #2, do you make it open for exploitation?
Right now, I don't know that you can even make it equal to THF unless you have special circumstances e.g. a rogue with lots of sneak attack damage and bonus weapon damage. But that's the problem. If you make it so TWF is just as good as S&B or THF, it's probably going to be overpowered for certain builds. You'd have to do something like nerf Sneak Attack or powers like it. Ask yourself if D&D is more believable where Conan does the most damage or where Cat Woman does the most damage?
That having been said, if I were going to make TWF playable for Rangers, I would give Rangers/Fighters Two-Weapon Defense as part of TWF. I'd also change Weapon Finesse for Rangers/Fighters to apply to all weapons and modify To Hit and Damage bonus regardless of the weapon type.
D&D, for reasons not entirely clear, really screws over Dex based Fighter/Ranger builds. Better armors reduce Dex benefit and there are numerous conditions where you lose your Dex
bonus, but almost none where you lose your Strength bonus or Armor bonus
Note that I don't think Non-Fighter classes need these benefits. Rogues already get tons of versatility and abilities. Making them the equal of Fighters for damage is fundamentally flawed since nothing allows Fighters to replace Rogues with regard to skills.