Almost nothing is known of the season, except that the series itself will experience a change, that the Winx will change their looks again (reminiscent of earlier seasons; like in Season 4) and will gain a new transformation. Currently, there is no information about their new transformations. The information can be verified from this site (in French): Saison 5, on Winx Magic sur Kazeo.
However, from the Licensing Zeigen 2010, via Michael's Winx Club, it has been confirmed that Roxy will be present, as the seventh Winx in the upcoming season. The quote is here: "A new Series Five, with further adventures of the seven magical fairies, is in production and will be ready in Fall 2011!"
It can be read here:Michael's Winx Club News Page. In 2010 it was publicly announced that Nickelodeon gained the rights for Winx Club in the United States, the United Kingdom, Latin America, Canada, Australia, New Zealand, Belgium, The Netherlands and Luxembourg and that it will be co-producing Seasons 5 and 6 along with Rainbow.
