Board Thread:Spoilers!/@comment-25703308-20150408091026

Lily.

Confirmed in Entertainent Weekly's website.

"Now that  Once Upon a Time has revealed the identity of Maleficent’s child to be Emma’s (Jennifer Morrison) one-time childhood comrade Lily (then played by Nicole Munoz), EW can now reveal that the adult version of Lily will be portrayed by Agnes Bruckner"

She looks the part, I think. 