Blizzard has announced a three-mission prologue campaign, titled Whispers of Oblivion, that will bridge the gap between Heart of the Swarm and Legacy of the Void.
More lore entries are out in the StarCraft II Game Guide.
On day two of BlizzCon 2014 (Nov 8th) the lead writer of StarCraft II: Legacy of the Void met with community members to discuss the StarCraft lore. From the official BlizzCon site: 10:00 a.m. - 11:15 a.m.: The Story and Lore of StarCraft II Find answers to your lore-related questions for StarCraft II with a little help from director of Story Development James Waugh and StarCraft II community manager Andrew "Arkitas" Dunne. Several topics were discussed and some questions submitted through Twitter were answered. After the formal part of this discussion James spent some time answering some informal questions. See StarCraft Legacy's forthcoming article on the Legacy of the Void Campaign items presented at BlizzCon.