If you want a campaign where medium armor is useful, try Arcanis. By adding Lorica Segmentata as a medium armor (+5 armor, +4 max dex, -3 armor check penalty, 20% spell failure, Special ability: still allows 30' movement), they made medium a very popular armor category. Essentially they attract the characters with enough dex bonus and movement features not to want fullplate but without evasion (which would commit them to chain shirts of some kind or other).
Based on that, I would say that the best ways to make medium armor useful would be to allow 30' movement or to increase the max dex. Kicking the armor bonus up to +6 for breastplates would also do the trick.
Without that, the kinds of optimal armor are much more limited:
Low level:
leather, mwk studded leather (for high dex rogues who don't want an armor check penalty)
Chain Shirt (for rogues who don't care about the check penalty, barbarians who want fast movement, and archers)
Chain Mail (for starting fighters with dex bonusses)
Splint Mail (for fighters, clerics, paladins, etc without dex bonusses)
Low-mid level:
leather, mwk studded leather (rogues who can't get a mithral chain shirt)
Mithral chain shirt (archers, rogues)
Chain Shirt (Barbarianas who want fast movement)
Breastplate (Fighters with mediocre dex going for spring attack)
Fullplate (Everyone with proficiency and low dex)
Mid level:
Mithral Chain Shirt: very high dex characters
Chain Shirt: low-mediocre dex barbarian types
Mithral Breastplate: Mediocre dex characters
Fullplate: everyone else
High level:
Bracers of Armor (Characters with very high dex and/or Bladesong/Canny Defense, etc)
Mithral Chain Shirt (Characters with modified 22+ dex)
Mithral Breastplate (Characters with modified 20 dex)
Mithral Fullplate (low dex characters with gloves of dex)
Fullplate (low dex characters who don't invest in dex items)