In a Medieval fantasy setting, you'd think think that half-plate armor would be fairly common, since different armor pieces could be mixed and matched, with full plate armor being rather expensive and hard to get (something only knights of renown, nobles, and big bad bosses would wear). As a former D&D’er myself, I like your idea of half-plate armor, and agree that there are some great Skyrim mods here on the Nexus for chainmail and great mods for plate armor, but nothing really for transitional/ half-plate armor.