To specify what I think a "sandbox mode" would be is something like allowing players to own official private servers where they're given creation tools to manipulate the environment, NPCs, items, etc. as they see fit. Sandbox servers would be solely inhabited by the player that owns it but other players can be invited up to a modest population cap.
Long ago I thought giving full map-editing tools to players wouldn't be worthwhile since it's separate from retail servers and thus inconsequential to the game. The success of Classic and increasing popularity of tabletop RPGs have caused me to change my mind. The former made me realize that WoW can be more than just a singular game and more of a collection of different game experiences under a single subscription. The latter made me realize how much appeal there is for imagination-driven adventures created by players themselves as opposed to fully produced adventures created by developers.
Basically, a sandbox mode can turn WoW into an engine for player expression, whether it be for aspiring game designers to create their own world on top of WoW or for dungeon masters wanting to create their own roleplay experiences with their friends. Classic already shows how people are willing to play WoW their way even if it has no effect on the live retail game. I've come to believe that a sandbox mode could be a natural next step to that, and open up a new avenue for community expression and growth.