I doubt DD2 would concentrate on multi-player. Most fans of the first 2 games prefer single-player, so I would guess it would have a strong single player game, and then an optional mutlti-player component (possibly a Battlefields type area, or a cooperative version of the main game). Most multi-player focused games are restricted to hunt and fetch quests, to keep the plot manageable for groups, which I doubt Larian would do to Divinity.
Full 3D means all objects and backgrounds are based on 3D models.
DD was all 2D, so characters and backgrounds were hand drawn and tweaked, and animations consisted of a series of frames. While many people like the look of the game, the animations require a lot of disk space.
BD has 2D backgrounds and 3D character models. Because the characters were 3D, there could be much more variety in character creation. There could also be more animations and more equipment, which would look different when equipped (in DD, all large shields looked the same, etc).
DD2 may have adjustable camera angles; that information has not been released yet. Lar has mentioned the base 3D engine, though: divinity 2, the 3D engine