The start of the gameplay is in an area we've seen before but I think where they go after that (Wyverns' Den/area before it) from 3:23:50, direct gameplay hasn't been shown of before. Link (from IGN's stream):
https://youtu.be/lH7FNHqM1Ps?t=12073
Update:
The Battle for the Heartlands army battle gameplay
The new area IGN demoed but with no commentary over it
https://youtu.be/lH7FNHqM1Ps?t=12073
Update:
The Battle for the Heartlands army battle gameplay
The new area IGN demoed but with no commentary over it