An example of one of these features is the 5e barbarian's unarmored defense. This ability, for those of you who haven't checked out the class yet, allows you to add your Constitution modifier to your Armor Class as long as you are not wearing any kind of armor.
While the realist in me knows this doesn't make much logical sense, the fantasy fan in me loves it. It gives me this image of barely clothed barbarians being so tough they can just shrug off a major blow from an enemy.
It allows you to create a barbarian character who'd be right at home in a Boris Vallejo painting without shooting yourself in the food mechanically. That's definitely a positive in my book.
Because I love this little feature so much, I've been toying with a way to give it to Pathfinder's barbarian for awhile. Here's what I've come up with after some brainstorming, too much free time, and an inability to sleep. I guess insomnia has its perks sometimes.
SAVAGE BOND (EX): At 1st level, a barbarian forms a bond with her inner savagery. This bond can take one of two forms. The first form increases the barbarian's base speed by +10 feet. This benefit applies only when she is wearing no armor, light armor, or medium armor,and not carrying a heavy load. Apply this bonus before modifying the barbarian's speed because of any load carried or armor worn. This bonus stacks with any other bonuses to the barbarian's land speed.
The second form allows the barbarian to add her Constitution modifier to her AC. This bonus applies even against touch attacks or when the barbarian is flat-footed. This benefit applies only when she is wearing no armor, light armor, immobilized, helpless, or medium armor, and not carrying a heavy load.
This ability would replace the barbarian's fast movement feature (the reasons behind that should be obvious), but still allow the player to make a choice. Also, I based the 2nd half of the ability on the monk's AC bonus feature, excluding adding the modifier to CMD because you can use this while wearing some forms of armor.
Furthermore, I should warn people this feature was made with the Pathfinder Unchained version of the barbarian in mind. The reason behind that is that version of the class doesn't grant a Constitution bonus while raging, which might break this ability. You might want to not allow that bonus to effect this feature if you want to use it with the Core Rulebook version of the class.
Currently, this is an alpha version of the class feature and still needs to be tested at the table. Some possible ways to reign it in if it ends up being too good would be to limit the specific amount of your Constitution modifier you can add to your AC, much like the duelist's canny defense feature, or maybe only allowing it to work with light or no armor. However, I'd like to see how this one works before attempting to patch it.