Blizzard just posted an update on Mercenaries, confirming it will be the final content update for the mode. They also shared a teaser of the next Mini-Set on Twitter.
News Post: https://hearthstone.blizzard.com/en-us/news/23892226/mercenaries-gets-a-mythic-update
Timestamps
0:00 Mercenaries is Dead
1:31 What Went Wrong
2:52 China’s Influence
4:19 Lost Opportunity
6:51 Too Little, Too Late
12:59 Other Modes
15:12 Mini-Set Teaser
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News | Hearthstone | Mercenaries is dead… Naxxramas Mini-Set teased!!