Fans of Pokemon X and Y should recognize the landmark featured in today's Smash Bros. Miiverse Pic of the Day: it's Prism Tower! Or, in more general terms, it's Lumiose City, the major hub of X and Y's Kalos region.
Sakurai, in his post, states that "up until a few seconds ago, the fighters were brawling on top of that tower", meaning that Smashers will get to duke it out on top of Lumiose City's Prism Tower in the 3DS version of the game; this also suggests that battles will also take place elsewhere in the city because the fighters are currently not brawling atop the tower. My guess is that players will also get to battle in the plazas surrounding the tower, seeing as they are illuminated.
As for hazards, perhaps players can expect to encounter cars and Gogoat tours impeding their progress in the match. What do you think of the stage? Excited? Not? Sound off inside! (There's also a larger version of the screenshot).