I don't think they would choose Juanita, because there seem to already be too many of those, with the King, Infanta Cristina's oldest son and Infanta Pilar's oldest son, as well as middle names for Pilar, Margarita and Elena's sons/grandsons. Federica/Frederika/etc probably not, since it is Victoria's name too.
I would have liked Eugenia, but Luis Alfonso has already used this, but I think Sofia, Maria, Lola or Gabriela would be very nice. Sofia, not only for the present queen, but for her grandmother, the Queen of Greece. That way, the two little Borbon girls born this year will both carry the names of their great-great-grandmothers.