For those who aren't familiar with it, Cupid and Psyche is the story about a mortal girl who falls in love with Cupid and has to endure trials set forth by the goddess Venus to be with him. Most notably, one of the challenges imposed on Psyche is to go to the Underworld, which incidentally, appears to be a storyline that is hinted on the show.
I think Cupid and Psyche will fit perfectly on Once, given the plot and themes in the story. It's about true love and a very much capable heroine willing to go through extremes to be with the one she loves. At one point, Psyche becomes victim to a cursed sleep and is awoken from it by Cupid! In the end, she is given immortality and joins the gods on Olympus with Cupid.
Given the parallels with Once and classic fairytales, as well as the Underworld quest in the story, I really hope the writers incorporates it on the show. Out of all the possible Greek myths, Cupid and Psyche makes the most sense in terms of Once standards. Since it is Latin literature, the writers can change the Roman names to that of their Greek counterparts to continue the trend of Greek myths.