Yoshinori Kitase, the director of LIGHTNING RETURNS : FF XIII responded a few days ago to this question:
"Do you think that the Coliseum will return?" in a exclusive interview with Lightning Returns team in Japan for the french media Website, Final Fantasy Dream.
The answer was very simple : "Yes something like the Colosseum of FF XIII-2 will take place in Lightning Returns: FF XIII, and everything will be detailed very shortly."
The interview of fifteen minutes will be available on their website very soon, with more details about LR : FF XIII, FF X HD, Type 0 and Versus !