If you're wondering why we've seen three Final Fantasy titles with the same main character (although she wasn't really even playable in FFXIII-2), it's because Square Enix wanted to extend the FFXIII saga.
This news comes courtesy of a new Famitsu interview (as detailed at Nova Crystallis ), and producer Yoshinori Kitase said that Lightning Returns represents sort of an "obsessive fixation on Lightning," which is part of the reason they're not calling the game FFXIII-3.
As for Lightning's new red-and-white outfit, which is apparently "quite revealing in the battle scenes," this reflects a certain history, in that Lightning now has the role of being a "liberator." That outfit was given to her by a god, apparently. In regards to gameplay, we will have free reign to explore and if we want to see everything, there will be a lot of "trial and error" involved as we only have 13 days (and each day translates to about two real-time hours). It's interesting to note that Square Enix "sensed the irritation" concerning that time limit. Said Toriyama:
"At the time of the Final Fantasy 25th Anniversary Exhibit, we showed the alpha version and as time rapidly decreased you could sense the irritation. We’ve since added a mechanic that will increase the remaining time in the world through quests and battles. By actively completing quests and battles, the time available to you will increase."
I just refuse to believe nobody at Square Enix could've predicted that "irritation" ahead of time. I just refuse to believe that. Oh, and lastly, they're saying the game isn't an action/RPG but mainly just an RPG with a few new action-based elements. …yeah, we'll see.
Related Game(s): Lightning Returns: Final Fantasy XIII