You'll meet up with Lightning again on the Devastated Dreams Map, and she will replace Hope in the active party, giving you access to a Commando. Set up your paradigm deck, because you're in for a big battle a bit later. Don't miss the Ember Ring which will be helpful a bit later. Shortly after the third save point, you'll face off with a boss, the Dreadnought. Upon defeating it, Lightning and Sazh will gain access to their final primary Roles, the Crystarium expands to stage 3, and you will gain the ability to upgrade and dismantle Accessories and Weapons (along with access to a new shop selling mechanical Components). However, you will not be able to backtrack from this point.