First, check out the shiny new Gamescom trailer for Capcom's Dead Rising 2: Off the Record, out October 11. Then take a look at the newly released Achievements list for the Xbox 360 version of the game, and the so-far-unannounced features they hint at.
The "I Got A Medal!" Achievement confirms the presence of a Sandbox Mode. This could be similar to the original game's Infinity Mode, which dropped Frank West into the Willamette Parkview Mall with no time constraints, a limited supply of food and a constantly depleting health bar.
The same Achievement also references mode-specific challenges, which players can earn medals for completing in both solo and co-op games. It also seems like there might be a new vehicle, since the "Cramped Quarters" Achievement is awarded to players for cramming eight survivors into a single ride, a feat which couldn't be accomplished in Dead Rising 2.