That Epic Tripping feat would mesh strangely with the Knock-down feat... either it would make it pointless, or the combination would mean that for every trip attack that hits, you do another trip attack (can you trip someone who's already been tripped?).

Some more epic feat ideas:

Epic Disarm: You may make a disarm attempt as a free action against any opponent who fails to hit you in melee combat up to a number of times equal to your Dex modifier. Requirements: Improved Disarm, Dex 17+

Epic Dodge: You gain a dodge bonus to AC equal to 1/3 BAB (round down) while not flatfooted, encumbered or wearing heavy armour. Requirements: Dodge (or any feat that counts as Dodge towards feat requirements), Dex 17+.

Epic Grapple: Count as if one size larger for all grappling checks and damage while grappling. Requirements: Improved Grapple, Earth's Embrace

Quick Thinking: You may add your Intelligence modifier to your Initiative bonus, and are never flatfooted against an opponent with less than half your Intelligence. Requirements: Improved Initiative, Combat Expertise