The new expansion of Hearthstone “Dr. Boom’s Secret Laboratory” was already hinted at in advance – namely in the “Witchwood”!
When Blizzard announced the new Hearthstone expansion “The Boomsday Project”, the surprise was great. But not for all players. Some fans had already suspected the upcoming expansion, as there was a big spoiler in the “Witchwood”. However, this was overlooked by most players.
Spoiler in Experiment 3C. The spoiler was hidden in a boss fight from the Witchwood and likely escaped the attention of most players. Occasionally, there is a fight against “Experiment 3C” in the Witchwood – players already know the creature from the card Nightmarish Mutation.
The witch Hagatha announces at the start of the fight that “Experiment 3C” is not one of her creatures. In the fight, there are then various hints that now have a clear context. The creature speaks of its escape from the laboratory in the Netherstorm. Only upon its death does the experiment proclaim that it is now “finally escaped” – a bit cruel and macabre.

Dr. Boom had to be played out. The conversation became really clear only when the player managed to play the original card “Dr. Boom”. Then a special voice line was played that is only heard then.
“Dr. Boom? Don’t bring me back to the laboratory!”
Of course, it was not possible at that time to recognize from this information that the next expansion would take place in the Netherstorm with Dr. Boom as the protagonist. In hindsight, however, this is an exciting detail that shows once again how often Blizzard teasers upcoming content already within the game.
This leaves the question of what little hints the community has not noticed yet.