The entire cast is tasked with transforming into Ragnarok as their focus. This includes Fang and Vanille, albeit 500 years before the events of FFXIII. That being said, no, you cannot fight Ragnarok as a boss or an enemy encounter.
However, as with the last few installments of the series, there is a chance for an international version to come out with added content and more endings. It is possible that Ragnarok may appear as an optional boss fight, though it is not likely.
Not at the moment, and after what happens at the end, good luck fighting it.
Bartie said that the sure way to kill fal'Cie is to transform into Ragnarök. That means the huge fal'Cie Titan too. I doubt that the gang could take down him, let alone something more powerful than Titan.
The International version already came out a while ago and I hear it has no additional bosses and the only addition is the easy mode.