Boss Monster 2 is an expansion or stand-alone set that is set to be released sometime in late 2015. It will contain all new Bosses, Rooms, and Spells, but it can also be combined with the original Base Set. The set will be called Boss Monster 2: The Next Level. Every card included will have a card identifier start as "TNL".
The kickstarter campaign has begun. Visit Boss Monster 2 Kickstarter for more information.
Look at Category: Boss Monster 2 for a list of cards.