Primarily, Disney via the Lucasfilm Story Group...
The Lucasfilm Story Group is a division of Lucasfilm, created after the Disney acquisition; it's their job to keep the Star Wars canon as internally consistent as possible, and to guide its development.
Pablo Hidalgo, a member of the LSG and one of the "continuity trio" sub-group, commented on Twitter that much of his job involves high-level storyline planning, and making notes on new in-progress stories:
@MisterCHolocron What exactly does being creative executive entail? I assume it's basically being an editor of sorts for books?
@pablohidalgo planning storylines, all content, reading/approving/commenting on scripts, manuscripts, outlines, etc. etc.
...but writers have the freedom to tell the stories they want to tell
However, Lucasfilm affords a great deal of creative leeway to the individual creators; as a Slashfilm article reports, based on the LSG live panel from Celebration Anaheim 2015, and as Hidalgo notes on Twitter, the Story Group doesn't dictate stories, just provides makes sure stories are being told at the right time:
The first point made at the panel was that the Story Group has a long game plan. They have major beats and ideas spanning probably years or maybe decades in advance (Note: no specific time frame was given) so as to be able to incorporate everything Star Wars is becoming.
While the term “Story Group” may make it sound like they are dictating the story, that’s not usually the case. In most cases, authors or filmmakers who are hired to work on Star Wars properties, are given the oppurtunity [sic] to come up with their own stories and ideas. While doing that, they work closely with the Story Group to make sure the stories are authentically Star Wars (for example, Darth Vader doesn’t dance or something) and also fit into the larger plan. Hidalgo said they’ve heard pitches from people who have great ideas for characters who are then told, “Come back in six years.” That’s because the story they’re pitching would fit in better with a character at another point in the future.
@bishbashboshjt Do LFL tell novelists the plot or do they Vet it with you?
@pablohidalgo We don't tell them the plot. It's their stories. We just try to make sure it's the right story to tell at the right time.
Although the LSG has a long-term plan for how the Star Wars universe will unfold, the fine details are left in the hands of the individual creators (in close consultation with the LSG, to keep the canon consistent).