If you're worried about whether the new heroes and maps added in the Eternal Conflict event for Heroes of the Storm will disappear once the event comes to a close, then your fears are unfounded.
However, beyond September 8, you won't be able to get Diablo as a free playable hero any longer and you won't be able to complete the quests and achievements related to the event. So no more "Kill 50 Treasure Goblins" or "Kill a Treasure Goblin" quests.
The event introduced two new Diablo-themed maps, the Infernal Shrines and the Battlefield of Eternity, as well as the Treasure Goblin mount. Players who achieve a player level 12 or higher in Heroes of the Storm will be able to unlock a pennant and portrait border in Diablo III. Just as well, you can unlock Malthael's Phantom mount to use in Heroes of the Storm by leveling a character up to 70 in Diablo III's latest season.
If you want your goodies, be sure to log in to Heroes of the Storm and play the game religiously before the deadline.
There's a good chance that a StarCraft-themed update is coming next, especially with the release of Legacy of the Void looming on the horizon. Just be sure to grab your Diablo goodies before then.