Pablo Felipe Juan Carlos Jésus Maria Alfonso Miguel Constantino de Los Todos Santos
Pablo (a good name for a king and after his greatgrandfather Paul I of Greece),
Felipe (his father)
Juan Carlos (his paternal grandfather)
Jésus (his maternal grandfather)
Maria (for catholics)
Alfonso (named after the former spanish kings)
Miguel (a nice Spanish name)
Constantino (his great-uncle Constantine of Greece, could be a perfect godfather)
Los Todos Santos ("of all saints")
If it's a girl i still prefer Sofia, cause it's a beautiful name and an honor to her grandmother Queen Sofia. But i also like Isabel, wich honored the earlier Spanish Queen, Isabel of Castile and Isabel II, who ruled in the 19th century. She won't be a queen, cause the Spanish trone is only for males, but if she became Queen, then she will be Queen Isabel III.
--> Isabel Sofia Letizia Victoria Eugenia Christina Maria de Las Mercedes
But I don't know any names of Letizia's family, only her father Jésus.