Some ideas, none of them playtesting, and perhaps lacking balance (Not all of them are related to a single weapon)
They need still some efforts to flesh them out, I think. Call it "Brainstorming"
Alternative One with his/her weapon :
Prerequisites: Weapon Focus, Specialisation, Improved Critical with Weapon, BAB 10+
When someone tries to disarm you (as return attempt or as "usual"), you gain a +2 (or +4?) bonus on your defense againt the attempt.
Fast Strike:
Prerequisites: Weapon Focus, Specialisation, Improved Critical with Weapon, BAB 10+
When you use your weapon one handed and you do not wear something (weapon, shield) in your offhand, you may strike as if you had the Ambidexterity/Two Weapon FIghting feat and would were the same weapon in each hand.
(So, a Fighter wielding a Long Sword in his primary hand and no shield, could make two attacks with the Long Sword with full of its regular bonus, but with a -4 penalty on both attacks. Advantage: You don`t have to buy two expensively enchanted weapons. Disadvantage: You won`t hit very well ...)
(It is, so to say, a "Flurry of Blows" with a "real" weapon)
Greater Disarm:
Prerequisite: Improved Disarm, BAB 8+
When you deal more than 10 points of damage with one attack, you gain one free disarm attempt at your current attack bonus.
You can use this feat once per round. (This is the "Knockdown" Variant)
Alternative Version (same Prerequisite):
If you succesfully disarm your opponent, you gain one extra attack at your current base attack bonus. (This is the "Improved Trip" Variant)
Greater Bullrush:
Prerequisite: Improved Bullrush, BAB 3+
When you deal more than 10 points of damage with one attack, you can make a free bull rush attempt (without the need to hit the target). You can only throw the target 5 feet back.
Alternative Version of Greater Bullrush (same Prerequisite):
When you succesfully bullrush an opponent, you gain one attack of oppertunity against him, as if he was moving through your thread range.
Supreme Whirlwind (or greater Spring Attack)
Prerequisite: Whirlwind Attack, Greater Cleave, Greater Spring Attack
When you are in light or medium amor, and on light or medium load, you can make a supreme Whirlwind.
You may move your base speed and still attack at your regular number of attacks. You suffer a -2 penalty per every 5 feet after the first 5 feet you made. (So, you could attack with your first attack at full bonus, move 5 feet, move at your next attack bonus, move 10 feet, and attack with your next attack bonus -4. Seems not work well, so far...)
Power Strike
Prerequisite: Weapon Focus, Weapon Specialisation, Improved Critical with Weapon, Power Attack, BAB 15+
When using the full attack option, you can give up your regular number of attacks and make a single strike. If you hit, you inflict damage as if all attacks had hit. If you hit critical, you deal only extra damage for the first attack. Sneak Attack damage applies only once. The damage is added together for purpose of massive damage death rules, hardness and Damage Reduction penentration and so on.
(Probably to deadly...)
Improved Charge:
Prerequisite: Power Attack, BAB 3+
When charging, you gain a +2 bonus on your damage roll.
Mustrum Ridcully