Well, I didn't have one specific setting in mind per se (since this was a bit of a test run to see if such a system would even work) but lets take two fairly usable examples.
Masque of the Red Death. A modern(ish) take on Ravenloft set in the 1890's (or thereabouts). The style of game can run from being very Victorian to Western to Southern Gothic. The important thing is that nobody wears armor; it doesn't really fit the feel of gentlemen, mediums, and charlatans that Gothic Horror invokes. In MotRD classically, there were four classes (replacing Fighter, Thief, Cleric, and Mage) so no worrying about barbarians or monks, but firearm combat is common, magic is rare(r), and monsters straight from the MM (like vampires, golems, mummies and werewolves) are a must have.
If the system worked, I wouldn't mind extending it to d20 Modern like 5e; set in 21st century Earth with magic and monsters. Same premise as MotRD, but more keyed to modern horror/fantasy (Buffy, Supernatural, Sleepy Hollow) vs. Victorian. A system that could make the transition would be excellent.
More to the point, I was looking for something that could mimic the
Defense Bonus mechanic of 3e era d20 rules. Since all classes have the same Bab, it seemed to make sense that they would all use the same for AC in light of lack of armor. However, people have pointed out some of the potential pitfalls (and that's not even addressing magic items).