I know it's not what your asking for, but I've been toyed with the idea of, if a player takes a feat that provides a +1 to a stat they already have at 20, the player gain the first of the following to apply:
[1] The player gains proficiency in a skill that is associated with that stat
[2] If the player possess proficiency with all skills associated with that stat, s/he selects one of those skills and doubles their proficiency bonus for any check you make with that skill. This can not be applied to any skill that already has to proficiency bonus doubled.
[3] The player finds themselves sol for that part of the feat.
To not have players choosing the feats for the stat bump.
The way I see it, there are 4 options when Ability Score Improvement comes up. A player can choose one +2 boost (to anything 16 or lower), two +1 boosts to anything (typically with at least one going to a 17 or higher), a specific +1 boost and a half feat, or a full feat. Personally, I feel it was easier (and slightly weaker) to have the +1 boost baked into the half feats, then list half and full feats separately and state you get a +1 of your choice in addition to a half feat.
In the game I am currently playing in, I'm a Charlatan Lore Bard. I took
Actor and
Master of Disguise (UA) feats. Between the two of them, after a day or three of observing and researching a target, I pretty much just get to walk into their stronghold/base/store and walk out with at least something my party was looking for. Yes, the two of them I grant me a +2 boost to my Charisma, but that's just icing on the cake I already wanted.
If I was building a non-Hexblade, Blade Pact Warlock and gave myself poor Dexterity, the boring
Moderately Armored feat would be an acceptable means to increase my AC, as well as potentially boost my chance to hit and deal damage. While the feat itself does nothing spectacular, it would help make my build less squishy without being forced to play a hexblade. Which allows me to have more fun my way. Now of course if you deemed to house-rule that I could simply spend a number of days of downtime and gold to gain the proficiency in higher tier armors, that feat would be potentially useless. It would really depend on how many days needed to be spent and how often they were provided.