Heroes of the Storm has three panels overall, where developers will talk about the new heroes and battlefields, and also share their thoughts on the current state of the game.
It's safe to expect that the opening ceremony will include the announcement of a new gaming event that is similar to the "Eternal Conflict," a Diablo-themed event in Heroes of the Storm, released on June 30, 2015. This would entail the introduction of a whole heap of new content dedicated to certain themes. The most suitable choice for this is considered to be the universe of StarCraft, which is honestly lacking in the current Heroes of the Storm build. Perhaps the developers will show some sort of high-tech battlefields or even early versions of famous characters.
It should be noted that if Blizzard will announce a new game event, it is most likely that it will be released no earlier than 2016. In this regard, it is possible that the developers will share some of their plans in the coming months.