Mass Effect 3: Leviathan producer Billy Buskell has taken to the BioWare Blog to share some of the experiences that he and his team had while creating the single player DLC. It's spoiler-free from what I can tell, but I'll still avoid reading the quote as I leave it for you below:
Because of the mysterious nature of the Leviathan, the team also started to think of how we could craft that mystery in the game. How could we have the player solve a mystery? We wanted the player to do the solving for themselves. This led to the idea of finding clues within the pack and that these threads would then lead the player to a central location where they could piece the puzzle back together, knowing there may always be more pieces to discover. In Leviathan, we give the players a brand new way to locate clues throughout their journey and to use those clues in a new in-game system to help track down new locations to visit.
This led to our next goal. Exploration was going to be a major part of Leviathan. In any BioWare game, there is exploration, but we wanted to take this idea even further in Leviathan. Exploration has typically meant creating additional space in a location or environment where the player can spend some time, find some cool loot, learn more about the story and universe, and trigger new quests within the larger story. For Leviathan, exploration meant adding more of this but also having a variety of diverse locations to travel to, and those locations would then present new areas to explore: everything from searching through facilities, vertical movement throughout a level and combat, and, for the first time in the Mass Effect series, controlling your movement in an underwater environment.