Originally Posted by Thomar_of_Uointer
D&D is already designed that way. Endurance>Diehard. Combat Expertise>Whirlwind Attack. E6 has Ability Training>Ability Advancement.
As long as your prerequisite feats are tolerably functional (+2 to a save is nothing to sneeze at) I think it's okay. The primary concern is balance, making sure that players don't outpace one another.
It's fine if you write new E6 feats that have 6th level or 6/9 skill ranks or a base save bonus of +5 as prerequisites, as long as you cater to all of the character builds your party has.
One option the GM could implement is write feats that have specific quest or money prerequisites. Want to take a feat to get wings? Okay, you have to perform a quest for an elder air elemental, spend 4,000 gp of reagents and spell components, and perform a ritual on the highest peak, and then the next time you take a feat you can get flight.
The trouble with that is Magical classes don't need homebrew feats, their standard feats are already awesome, while the majority of combat feats are terrible.
So the deck is already stacked against physical characters.