Ocular surgery that adds resistances or prevents or reflects gaze attacks?
Gastric surgery to remove the ability to be sickened. (There is a nerve in the stomach that can be severed, and it prevents nausea and a lot of dizzynes, but removed the ability to purge, so it's dangerous to sever in heavy drinkers.)
Various piercings about the body that may reduce Con, but increase Str or Dex?
Houdini-esq hiding of small things under the skin?
Cultivation of parasites that offer strange benefits? (Lice that inject a toxin that reduces the effectivity of nerve endings, which reduced damage from attacks by 1, or mites that eat away at dead flesh to inspire organ repairs increase nightly regeneration by 1 or two points.)
Constant intake of hallucinatory substances to get a bonus against illusions, possibly narcotic substances to get resistances against enchantments?
Attacking key muscle structures in grappling creatures to reduce their ability to grapple?
Nasal surgery to produce a sound that vibrates on a level that confuses creatures with Blindsense?
Edit 1: Another Thought
Gastric Surgery to use a vomit attack that deals acid damage, and may cause nauseated or sickened condition.
Development of reflexive ear wax??? that helps protect against language depended abilities?
Subconscious training inorder to instill a 'contingent' suggestion for whenever failing a save against suggestion or dominate spell. I.e. Sit down and repeat the Mantra 'The Books with odd looks are most often written by Crooks' whenever I fail a suggestion or dominate spell.?
Swimming out of a slime/ooze with relative ease?
Adding a mild or an ineffective disease to his blood that harms certain creatures maybe, that succeed on bite attacks?
Eating garlic or some other potent herb to make creatures with Scent feel uninclined to be within a certain distance?