Winx Club Forever
It is possible that once Witches graduate from Cloud Tower they also become Guardian Witches since, according to Griffin, the witches also fight evil, except that they use dark magic while fairies use light magic.
Know Guardian Fairies
The Winx graduate (except Bloom) and become full-fledged fairies and by becoming guardians of their home worlds they are more connected to their magic and it becomes powerful just like them. At the end, Bloom fully complete's her Enchantix and becomes a Guardian Fairy.