Everyone has seen the Achievement feats in LoFPG (and if not, go and take a look!) and everyone agrees that they're great (and if not, you're just wrong and should change your mind! ;-)), and I'm sure we all want those for our other APs now, too!
Let's brainstorm what would work for second darkness.
Remember: The first rule of Brainstorming is to openly mock and ridicule the opinions of others. ;-)
I'll start. Not all of them might have everything there, sometimes it's just an idea about what you need to achieve before you can get the feat, and suggestions for benefits are welcome!
Spellbreaker[Achievement]
You have fought against creatures resistant to magic so much that piercing their resistance has become a matter of course to you.
Requirement: Succeed at 25 spell resistance checks delivering harmful or offensive spells.
Notes: The number of checks you have to make could be changed, and I'm not sure about the benefit. Maybe a +2 to spell penetration above and beyond spell penetration and greater spell penetration, and/or the ability to take 10 on those checks, maybe at least x/day, where x can be a flat number like 3 or your spellcasting key ability score.
The following feat is kinda spoilertastic and should only be revealed to the players after Shadow in the Sky has run its course!
Kinslayer [Achievement]
You have made it your life-task to cleanse the world from the foul blemish that is the elves dark skinned kin, the Drow.
Requirement: Elf, deliver the killing blow to at least 20 drow, driders, or demonic minions of the drow (familiars do not count).
Benefit: As long as you carry a sign of the elven nations or shout an elven war cry at the start of a battle, you gain a +2 morale bonus on Will saves. YOu also gain a +2 competence bonus on attack rolls and weapon damage rolls against, drow, driders, and demonic minions of the drow (including familiars)
Notes: Of course, this is similar to All Gnolls Must Die, and I think there can be more feats like that for most adventure paths: SD could have one for Aberrations ("Hunter of Abominations"), RotRL could have one for Giants ("Jorgenbane") Second Darkness maybe one for Red Mantis (though that's probably too limited)
The following feat() can work in more or less every campaign:
Pyromancer[Achievement]
Your obsession with the element of fire has attracted one of the more destructive elementals spirits of fire, who infuses your fire spells with extra force.
Requirement: Deal a total of 1000 fire damage to creatures. Cold magic displeases the fire spirits, and every 1 point of cold damage you deal reduces your cumulative damage total by 1.
Notes: There are many possible benefits for this feat: Increased caster level, increased save DC and attack bonus if required, even cheaper metamagic.
Another possibility would be a secondary effect to all spells that deal fire damage: Victims cathing on fire on a failed save, or extra non-lethal damage as the fire evaporates your internal water reserves, adding the sickened condition as the burns continue to hurt the victim(maybe grant a will save to negate). Duration based on damage dealt
This sort of feat isn't just for fire magic, of course!
Cryomancers (focus on cold, ban on fire) might give victims penalties on speed and/or dex-based rolls as the cold stiffens you (fort negates).
Aeromaners (for lightning against acid) might stun enemies, or at least lower their AC)
Terramancers (for acid against lightnint) might have lingering spell effects.
Bump
Never Take the Antidote before the Poison
Your encounters with darklands foes and venomous monsters have to taught your body to resist toxics and poison
Requirement: You must have failed 20 Fortitude saves against poison.
Benefit: You gain a +2 bonus on all saving throws against poison. If you fail your initial save against poison, you can, as an immediate action, gain the benefits and drawbacks of the Rage spell for one minute or until you are required to make a Fortitude save against secondary damage. You can only gain this benefit once from any single attack. You cannot gain the benefit of this feat if you are already taking damage from poison