I'd love to see DR in heavy armor
please please please Wotc let's make this the edition that gets heavy armor right. Not in an optional module, but a simple, easy, quick, rule that is balanced and fun. I agree 100% with the other posters here saying there should be mechanical advantages to each armor up the line, not just financial ones to consider. I actually quite dislike not being able to wear plate at all unless trained. If the suit fits, wear it!
One EASY way to balance heavy armor vs light, is to make it give -1 or (-2 untrained) to all attacks. You are treated as "clumsy". Then, as you progress as a fighter or with a heavy armor theme, you can overcome it. Has no one here ever played Pathfinder with Armor Training 1/2/3/etc for their fighters? Your max dex penalty in heavy armors goes away eventually.
One way to do DR is to just add HP to your guy when he wears armor. It's simple, effective, much easier to balance. This armor grants you +20 HP to your max when you wear it. Or whatever it is. It keeps combat quick, simulates DR without the extra math, and is way more granular. You still take that 1hp damage but it counts less. Granularity is good. Then your magic dragon plate can give you +50hp instead of the usual +20 for regular plate...opens up a wealth of more possibilities for magic armor.
I'd actually be OK with light armor+high dex users having close to the AC as heavy armor, if DR or extra HP as DR system were used as well. It would allow a ghoul touch to be avoided/resisted just as easily.
Fighters should get two themes, that they can use for offense or defense-oriented themes, rangers can pick two from weapon styles or rogue styles or druid styles. A defense-oriented fighter could pick the heavy armor training to either allow more of his dex bonus in heavy armor, mitigate the -1 attack penalty I just proposed. You are a tank...you're slow..at first. Later on you become a master and just as nimble as in your birthday suit. After many years....
The best way to balance plate IMO is to make it uber defense / ac / dr/+hp AND give it a penalty to hit. Maybe plate gives -2 to hit, which, as we know in this edition, means a lot. If a wizard takes a plate-wearing theme...and eventually gets a light-magicky plate armor and has high enough level in his theme...he should be able to cast wearing magic plate just as a fighter should be able to jump and climb in it, i.e. quite well. Magical plate should be, well...magical.