There are apparently two more to come. I think it's possible to flesh it out, the plan is clearly to link the Hobbit and LOTR far more deeply than was originally the case. The Hobbit was written first, and then Tolkien expanded the world for LOTR.
The Brown Wizard is Radagast, who has a bit part in the Fellowship of the Ring and possibly the Hobbit too, though it's been a while since I read it. He's there to help link the two trilogies as they're going to focus a lot more on where Gandalf went while the Dwarves and Bilbo travelled through Mirkwood.
Whether or not this attempt to deepen the story will work we'll find out. I think that they may find it a stretch for a film, and yet bizarrely part of the reason for the length is that they've missed little if anything out and put stuff in! By contrast LOTR skipped absolutely loads -- many of Frodo's adventures travelling to Rivendell (Tom Bombadil), for example, and what happened after.