defensive duelist i'd make DEX/CON, given it's meant to be a defensive option bumping CON seems like a good shoring up of your vitality if an attack does get through your AC boost
I went with STR/DEX since those are the two weapon attack ability scores. You're either parrying via deflection or block/ guard. The former being DEX-based, the latter STR-based IMO.
The point of the feat is to
not allow the attack to get through, not shoring up hit points in case it does.
However, for simplicty and to make things more versatile, you could probably group feats into three (maybe four) categories: physical, mental, and neither (and maybe any). Any "physical" feat could be STR, DEX, or CON ASI, a "mental" feat of course would have the options of INT, WIS, or CHA for ASI. The "neither" group wouldn't have any ASI, and if you include the fourth group the ASI would be floating.
personally i don't see the issue giving prodigy a floating ASI, it's a species feat exclusive to three species choices, it might outclass skill expert in the specific circumstances when in consideration to those three but isn't that part of the nature of making restricted options vs more available ones?
I actually considered this... Currently Resilient, Skilled, and Skill Expert (though I don't like it) are the only three that
basically allow a floating ASI.
However, in keeping with my personal philosophy that any ASI should have a tie in to the feat it comes from, such as Resilient granting the ASI to the saving throw proficiency gained by the feat.
Although I think Skilled is a very nice feat, I know a lot of people don't consider it very strong, so I added a "floating ASI" to that one, but really I would prefer the ASI to be linked to one of three skill proficiencies gained (CON being the only other option since it has no linked skills).
I would also prefer if Skill Expert worked similarly. Pick a skill, pick a skill for expertise, gain ASI in the ability linked to either the skill or expertise granted by the feat. (Again, CON could be an exception as an addition choice since it has no linked skills.)
Prodigy (as shown) has the race prerequisite, but honestly from day 1 we houseruled removing that--it seems a silly and unfounded restriction. Why should only human and part-human have "prodigies" in their race?? So, I debated including it in the draft above since we don't use it, but decided ultimately to leave it in to reflect a more RAW-view.
If anything, I would have the ASI for Prodigy be linked to either the skill or expertise, with CON an extra option as with the others.