Final Fantasy XIII-2 is going to feature a darker story than the last game, as if you can get darker than a group of terminally ill people with absolutely no hope for their future. Producer Yoshinori Kitase and director Motomu Toriyama have also said they listened to XIII's critics and will address some issues.
In XIII-2, Lightning and her cadre of impossibly-haired friends will be searching for a gate to an invisible world known as Bunibelle and they'll be meeting Etro the Goddess along the way. So far, so sensible. Characters from FFXIII will return, with some "unexpected" changes.
The battle system has been "evolved" but Square Enix isn't spilling details just just. We do know that the characters, story and interface are all being developed with criticism from both Japanese and Western gamers in mind.
Sounds interesting, but we'll see. I thought XIII itself was already too dark, humorless and miserable, so I don't know if going even darker will help.