To contrast the opinion that a previous asker had about magic making too many doppelgangers, I really like the effect and don't feel it to be over powered.
When I started playing magic, around M13, I was in college and didn't have much disposable income. It seemed like the "cheap" cards I could afford were all clone type cards and cards that could cast my opponents instants and sorceries. I ended up getting so many cards I made a Wrexial, the risen deep EDH deck. I would commonly get asked "How good is your deck?" My reply was always, "I don't know, how good is YOUR deck?" It carried me a good long time and provided lots of fun for myself as I was trying to figure out how to essentially win with their deck and a fun challenge for my friends as they had to play around their own cards.
TLDR: please don't change how many doppelganger/copy style cards you're creating.

How do others feel about cards that copy the opponent’s cards?

Tis my annual day of b-day asking.. what you can tell us about the design of Yeva, nature's herald my favorite elf commander ?

Yeva was part of a rare monocolored legendary creature cycle in a Magic 2013. I believe those were the first legendary creatures to appear in a core set. Happy Birthday!

What's the difference in approach between M13 being Bolas themed and M19 being Bolas themed?

The difference is 6. : )

I would love it if that feather was one of Avacyn's because of Tibalt becoming extremely powerful and taking over the plane! I have a feeling it has something more to do with Liliana or some other villain however. Thank you for returning to this set. I started just before RTR was released so I made my starting decks out of packs of Innistrad and Magic 2013. Perhaps we can see a return of Odric?!

We’ll have to wait and see. : )

Some AS-FAN trivia for you! Fate Reforged has an AS-FAN of 0.666 Dragon creature cards, meaning on average, 2 of every 3 Fate Reforged boosters have a Dragon. The previous high as best I can find is a tie between Magic 2013 and Magic 2014 at 0.174, or just a little better than 1-in-6. Scourge, the previous "dragon set" had an AS-FAN of only 0.091 (1-in-11)! For reference, Avacyn Restored has an Angel AS-FAN of .448, or just a little worse than 1-in-2.

FYI.

"We did not know what the Theros card would be when Magic 2013 was being done. Heck, we didn’t know what it was going to be when the art of Theros was being done." So wait, how do you make sure there aren't too many redundant common effects in a given Standard season at one time? And even if the m13 team didn't know, wouldn't the Theros team notice there's a spell that does what their spell does? Just for the record I'm not like *mad* just curious. I *like* two-mana Instant burn spells.

In design & development, we can change the present and the future but not the past. Once something is printed (or past the point where changes can be made) we have to live with it and design/develop around it.

Garruk, an "beefy" planeswalker? In Magic 2012/Magic 2013 he looked like an unhealthy roid-builder, ready for a deadly heart attack. More realistic muscles in MtG!

Remember, this is fantasy and he’s also eight feet tall.

I was surprised State of Design 2012 made virtually no mention of Dark Ascension or Magic 2013. The interesting points to me would be: why was Dark Ascension so much less successful than Mirrodin Besieged? And: What's the overlap between Innistrad and M13 Limited that makes both so good?

I just ran out of space. Dark Ascension wasn’t less successful than Mirrodin Besieged. Both Innistrad and Magic 2013 had excellent design and development. That’s usually the key.

RE: "Multiverse dumps" and my last question. I do indeed mean the developer comments for cards from the Multiverse. I may have just missed the ones for the INN block/M13 then, I'll have a look around on the Wizards site. It'd also be very interesting to see them for older sets though, assuming the comment files still exist.

Dev comments, as we call them, are usually saved for “Latest Developments”, the development column on Fridays.

How much of m13 feedback can you use on m14? Is it too late for the next core set once the current years set is released?

It’s late but not too late.