Hard to say, because he replied to both Keegan and the fan, there’s nothing to indicate whether he just meant the love story plot or the Depp part but I agree, there’s far too small a chance of them snagging Depp.
ALthough, Depp goes in for quirky characters, and OUAT has the potential for some VERY quirky ones, like say, playing the Sarlacc….an alien entity composed of the consciousness of various absorbed yet still alive, slowly suffering victims? I could see Depp signing up to do something crazy like that just for the challenge of it.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass