Compliments of Nintendo World Report, the European box art for Luigi's Mansion: Dark Moon 3DS has been revealed. Located in the top right corner of the box is the Nintendo Network logo, which denotes some sort of online functionality.
The only problem is, we don't know exactly what "Nintendo Network functionality" entails yet. It could be anything from DLC mansions, to online leaderboards, to potential online mini-games.
Considering Square Enix has already ushered in a new era for the 3DS with Theatrhythm DLC (and the fact that Nintendo itself will utilize DLC for NSMB2/Fire Emblem 3DS), anything is possible.
Let the speculation commence!
Luigi's Mansion: Dark Moon Features Online Functionality [Nintendo World Report]