Bluetracker
Tracks Blizzard employees across various accounts.
Blizzard hinted at the two most recent expansions 3 years ago
If you look closely at the image for Far Sight, you'll notice two things: 1) the background seems to be a crater of some sort with jungles and waterfalls (Journey to Un'Goro) and 2) in the eye you'll see Ice Crown Citadel (Knights of the Frozen Throne). I guess this card really did have Far Sight.
Iksar
The process has changed a bunch in the last 3 years, but generally we all go into a room and jam ideas onto a white board to try and see what everyone is most excited about making. Blackrock Mountain was the first time I remember doing this, and it's been on the white board as an idea ever since then.... it's just taken awhile to get there.