My opinion here...
FFXIII does NOT need a sequel. It needs a sequel like FFX-2 needs a sequel. Or FFX for that matter (shivers the mention of the atrocity that is FFX-2). The story ended with the bad guys gone, Cocoon saved, etc. etc. I have completely lost all faith in the FF series at this point. -sighs- I'll probably still look at the game because I don't like to judge before seeing, but Square's pretty much dead to me after what they did to Parasite Eve.
An FF game that needs a sequel? FFVI. Kefka succeeded. He DESTROYED THE WORLD. If an FF game needs a sequel, its that one.
Or FFIX. The world wasn't destroyed, but Bermecia was destroyed, Cleyra was blown up, an entire world's worth of Genome's now has to make their home on Gaia, Terra was destroyed, Alexandria was blown up, there was lotsa war, they still have the remnants of the Mist to take care of, people have to figure out how to live without the mist that served as energy for near everything, etc. etc. Not as much as FFVI, but it is certainly more deserving than FFXIII. -sigh-
Just my opinion.