I'm a bit underwhelmed with the strikeback feature, read as written. It seems as though a group of people will delay their turns, readying actions to hit an attacker with reach, not knowing whether or not that attack will come.
The way I would reword it is thus:
You can ready an action to make a melee attack against any foe who makes a melee attack through a square that you threaten, even if the foe is outside of your reach. If the foe takes a standard action to attack you, you get a standard attack. If the foe takes a full attack option to attack you, you get a full attack option to attack it.
As for Lunge, I would make it more in line with how the combat maneuver feats work: Anyone can perform a lunge attack, but they suffer -2 to attack and -4 to AC and incur Attacks of Opportunity.
People who take the lunge feat become proficient at performing lune attacks, reduces the penalty to attack to 0, reduces the penalty to AC to -2, and do not incur Attacks of Opportunity. Strike Back can be used against lunging opponents.