So according the reliable Italian news source MovieTele the Winx's second transformation will be called "Mythix" the name is nothing special but it also sounds like it will be interesting to attain.
Above is an image of the transformation, it does look a bit simple to me. Let's hope not all the Winx look the exact same though.
Italian:
Grazie ai consigli di Eldora, le Winx partiranno alla ricerca di oggetti magici e viaggeranno nel Mar dei Caraibi, nelle foreste canadesi
e in tanti altri luoghi meravigliosi, nel tentativo di forgiare la
chiave che chiuderà per sempre il Legendarium. Per riuscire nella loro
impresa, le Winx otterranno il potere Mythix col quale salveranno l'Universo Magico.
English:
Thanks to the advice of Eldora, the Winx leave in search of magical items, and traveling in the Caribbean Sea, in the Canadian forests and many other wonderful places in an attempt to forge the key that will close forever Legendarium. In order to succeed in their venture, the Winx will gain the power with which Mythix save the Magical Universe.
Here is the article source link since I was getting complaints about my news sources: Click here!