I've been trawling Pathfinder resources looking for interesting feats that I think will benefit Kingmaker (maybe in DLCs?). I've tried picking things that make for more interesting theory crafting.

Antagonize - specifically the Intimidate aspect; essentially making the user a high-priority target for a round
Breadth of Experience - a neat feat to give old races a bit of depth, the effect should just add +2 to all the Lore and Knowledge skills
Channel Smite - essentially a single-target channel; gives access to Guided Hand
Guided Hand
Improved Feint - Kingmaker does a great job of adding a wide variety of combat maneuvers; but I've been thinking for a while now that feinting is sorely missing in Kingmaker
Greater Feint - improvement on basic feint
Second Chance - a feat that makes Combat Expertise a bit more attractive
Whirlwind Attack - I was surprised to not find this feat in Kingmaker
Disruptive - together with Spellbreaker a great incentive to level fighter
Extra (...) - there is a whole range of Extra feats that would help (Extra Ki, Extra Rogue Talent, Extra Discovery, Extra Lay On Hands, Extra Mercy, etc)
Hamatulatsu - a splash of color for unarmed strike users
Monastic Legacy - another unarmed strike feat, makes monk multiclassing more attractive
Medusa's Wrath - the "capstone" for monks that reach level 10 (as it is a bonus monk feat available at that level)
Furious Focus - the feat that makes quick calculations on expected DPS with Power Attack harder
Shield of Swings - a cool defensive option for two-handed weapon wielders
Reward of Life - boost for Lay On Hands
Two-Weapon Defense - boost for two-weapon fighting
Unsanctioned Knowledge - a paladin feat that deserves to be implemented purely on the grounds of its awesome name
But a Scratch - the perfect feat for somebody like Amiri or Regongar
Improved Bravery - a fantastic feat to shore up poor fighter will saves
Inspiring Bravery - the feat that let's the fighter act a bit like a paladin/bard
Social Bravery - add some bluster to your fighter

Furthermore some style trees:
Boar Style (feat 1, feat 2, feat 3)
Jabbing Style (feat 1, feat 2, feat 3)
Snake Style (feat 1, feat 2, feat 3)
Tiger Style (feat 1, feat 2, feat 3)