I don't like how you're railroaded into going all-in on either Str or Dex, and Medium Armor is obsolete in the middle. Would be cool if high Str AND Dex was somehow viable as well as a more defensive and versatile build. Say a High Elf Fighter with high Str and Dex at the cost of lower Con.
5e could potentially just expand on the Str requirements already present in the system. Heavier armors still allow dex (with diminishing returns) but require more and more strength to wear and/or make use of.
A unified system could be something like: Every armor has a Str requirement (at least 10), and you can add up to [Str Score - Str Requirement]/2 points of your Dexterity modifier to your AC. E.g.,
- Light armors would have base Str requirements of 10, and thus would work as normal w.r.t. Dex bonus.
- Breastplate could have a Str requirement of 13, giving AC=14+[allowed Dex]. A character with 15 Str and 14 Dex could add 1 point of dex (15-13)/2=2, whereas a character with 17+ Str could add both points of Dex.
- Plate could require Str of 17. You'd only be able to add any Dex with a Str score of 19+
- Massive Plate could require Str of 19.
Idk, just some random thoughts.