The only playable characters that have been revealed thus far.
Lightning's younger sister, and Snow's fiancee.
She is in XIII , though unplayable, as she is the person Snow and Lightning are trying to rescue throughout the length of the game.
Now, apparently she is the one rescuing Lightning.
Not a lot has been revealed about Noel. The E3 demo showed Noel being extremely protective of Serah, which may indicate something. Although, a major part of the first game was the romance between Snow and Serah, so I don't think this will be changing anytime soon.
He will also have a more prominent role then Lightning in the last game.
Serah's sister. A line in the E3 trailer implies that Lightning dies at some point in the story, though i don't think she will.
She has also undergone an awesome costume change. (Compare this picture with Lightning in the video above)
Serah's fiancee. It is unclear what his role will be in XIII-2. I can only assume he will occasionally be a playable character.
Motomu Toriyama also commented on how Snow will react to Serahs new friend, and how much time they will be spending together. He remarked that Snow's personality isn't a jealous one, and that he won't mind at all.
Personally, i really hope Snow lands a serious role in the story. He is the sort that feels responsible for everyone, and always tries to lead in the right direction. To not give him an important role would be silly!
Unfortunately, nothing has been revealed on whether Sazh will be in the game. Hope either, but i only care about Sazh. We can only assume they won't, as Vanille and Fang will not be returning for the sequel.
As we all know, FFXIII was extremely linear for about 70% of the game, and then the remaining 30% was semi-linear. FFXIII-2 will have towns, and greater areas for exploration. Each person within the town will have it's own AI, and also mentioned was that the game will have a monster raising component, but that's all the detail given.
There will also be Side-Quests, which were lacking in the previous game, Mini-Games, and "Plenty of complexity".
The cinematics in Final Fantasy games have always been second to none. The amount of work and detail that goes into them is phenomenal. Well, adding to the amazingness of the cinematics, are cinematic action sequences. This will mean that during battle, occasionally an action icon will pop up, and if performed correctly, the strike will deal a substantial amount of damage to the enemy.
Also adding to the experience, will be dialogue trees. Occasionally, within a cinematic, a dialogue wheel will come up, similar to Mass Effect. This allows you to choose what dialogue a character will say. This will effect the story to some degree, but not by a lot. The game may have multiple endings, so perhaps the dialogue wheel will influence this.
The game's battle scenes will remain very similar to the first game, with some small changes
Enemies will no longer simply linger in an area, and pursue you when they see you. They will now spawn in front of you, with enough space to act with the Mog Clock.
The Mog Clock appears when you see an enemy, and will change color as you approach. This will determine whether who starts the battle with an advantage, you, them, or neither.
Fighting a monster will be a lot similar, with the Paradigm system still in effect. Upon defeating a monster, however, you will have a chance to adopt it into your party. It will then take up a character slot, and different monsters can be set to individual paradigms.
Lightnings summon "Odin" will make a return, however whether or not Noel has a summon is unknown. Serah will not have a summon, but a moogle instead.
Moogle's are small creatures that act as a lot of things, including scouts, and even transformational weapons. Serah can also take the monsters into her party.
Squenix have been keeping a tight seal on information regarding FFXIII-2. At any rate, it looks like it's going to outdo it's predecessor. I'm looking forward to it's release early next year (December this year for Japan), i've just gotta work on finishing the first one beforehand!
Below is my absolute favorite scene from Final Fantasy XIII. The scene Snow proposes to Serah and then they ride through the fireworks. The amazing graphics are after the proposal. Yes, i'm a gooey romantic, get used to it.
I would suggest watching in full HD to see just how amazing the cutscenes really are in XIII!