I will repeat my overall reaction to this episode (for a fuller explanation, see that).
I think the original guardian was meant to be Drazilla, but now her heart is too dark.
The Guardian is someone who is Pure-Of-Heart. This is someone who will suffer anything and everything for their family, over and over again. Right now, the best candidate is Regina. (Perhaps she has removed her darkness and purified her heart through her deeds in the new Realm??).