Wizards of the Coast has revealed more of its plans for Dungeons & Dragons product flow, including the number of storylines a year, its next two products, and plans for the Open Gaming License.  
During a Reddit AMA last week, Senior Manager for the Dungeons and Dragons Research and Design Team Mike Mearls ended a response to a  question regarding what kind of campaign books would be released in the future with this statement, "You can expect us to do one or two such products a year, to give people enough time to play through a campaign without overwhelming them with new options."
Mearls confirmed the schedule and revealed how far out the planning runs in an interview with Israeli website pundak.com.il.  "We're looking at two storylines a year,” he said.  "Right now, we have plans laid down for stories up through 2018."
The next storyline adventure books, The Elemental Evil Adventurer's Handbook and Princes of the Apocalypse, will release on March 17, and be produced by Sasquatch Game Studio (see "Sasquatch Launches with 'Primeval Thule'").  Info on the books was leaked via a post on ENWorld
According to the leaked info, The Elemental Evil Adventurer's Handbook is a Dungeons & Dragons accessory that includes player information for the Elemental Evil story arc.  It includes skills, abilities and spells as well as background, setting, and story information.  The hardcover has an MSRP of $39.95.
The Princes of the Apocalypse adventure book details a super-adventure for the Elemental Evil story arc. Intended for Dungeon Masters, the book details an adventure that sets heroes against the four corrupt prophets of the Elder Elemental Eye, who are constructing four elemental temples linked to the power of the elemental princes.  The adventurers must locate each prophet’s hidden source of power, and destroy it before the evil forces can enter the Realms.  The hardcover has an MSRP of $49.95.
The first storyline campaign books, Hoard of the Dragon Queen and The Rise of Tiamat were produced by Kobold Press (see "Exclusive Interview: 'D&D's' Liz Schuh on the New Edition Launch").  
Wizards of the Coast may not be the only company producing official products using the D&D 5th Edition game engine, according to D&D Story/World-Building Lead Chris Perkins.  "It is our intention to bring back the OGL," Perkins said of the Open Gaming License on a panel at Gamehole Con last weekend (as live-Tweeted by the convention organizers).  "We just don't know when we are going to do it yet."
Contacted for confirmation of these facts, a WotC spokesperson would only say, "I can only confirm that right now we are focused on our current storyline, Tyranny of Dragons."