I guess one question is whether he is a stranger to her or she knows him.
The forehead kiss suggests that he’s someone Emma is familiar with. Guess that doesn’t necessarily eliminate the possibility that he has a secret identity she doesn’t know about (maybe his name is Clark :-P).
Christopher Lloyd also played the WofOz and he would come and go from Oz to our world to help bring back Dorothy’s memories in Dorothy and the Witches of Oz.
And they’ve already used one Back To The Future shoutout for Emma’s NYC storyline.