There's of course nothing stating that next-gen games have to be 3D. But its kind of within the idea of next-gen systems; you upgrade the technology to allow for more complexity in your graphics/sound/interface/control scheme/etc. Of course, you'd also want video game companies to still deliver with excellent storylines and interesting characters, innovative concepts as much as possible. Typically however, the market is driven by old franchises being continued by sequels and massive upgrades in graphics.

That's not to say however that they all have to follow that line. A good example of a good 2d-ish game from what was previously a next-gen console was Paper Mario for the GameCube. Its certainly not a strict necessity that games by 3D for next-gen, but you'd figure almost all would be.