Craven is a popular one. +1 dmg/character level.
Gloom Strike - trade 2d6 to impede target's vision for 3 rounds (target has to roll vs 20% miss chance0
Dead Eye - add dex bonus to dmg dice - doesn't even require SA
I could keep typing, but instead go to
dndtools.eu where I've selected feats and added a filter of "precis" (for precision and precise). Click through the various feats that sound good. Replace "precis" with "sneak", "flank", "opportunity", "lose dex", and any other words that might apply. You'll have more feats to choose from than you know what to do with. Let us know what the best ones are.
Good luck!