Because of the name they choose for their first daughter, Leonor, is a historical name, a name from the Spanish Royal Family, I expect Felipe and Letizia to choose another such historically significant name for their second daughter. I made up a list of names of women from the Spanish royal family (only on the paternal line, following the crown, thus excluding the Greek royal family, etc)
Names that have appeared more than one time (with the number of times in parenthesis afterwards) (this was from an English-speaking hereditary tree, so the names are spelled in the English variation. I don't remember off the top of my head which ones were not of English basis to begin with (such as Mary, sister of Q Elizabeth I) so I haven't changed it)(*edit- I put the Spanish equivilants at the end of the list)
Anna (3)
Antoinette (2)
Catherine (3)
Isabella (11)
Isabelle (3)
Joan (2) (Joan? that seems odd to me..)
Josepha (2)
Margaret (7)
Maria (12)* (this only includes women who were simply named Maria. Fifteen other women were named Maria _something_)
Maria Christina (2)
Maria Anna (2)
Maria Theresa (4) (the name of the current Grand Duchess of Luxembourg!)
Marianne (4)
Marie (2) (these were specifically Marie, not Maria, as they were born French. All Marie's were specifically Marie. Otherwise, I list them as Marias)
Marie Christine (2)
Marie Louise (6)
Other names include Beatrice, Charlotte, Eleonor (hence, Infanta Leonor, the new baby's big sister), Eulalia, Louise, Louise Isabelle, Louise Charlotte, Mary, Mercedes (as in Maria de Mercedes, the king's mother), Theresa, and of course, Sofia, Elena, Cristina, Victorica, Federica, Letizia, and Leonor. This is not an absolute, nor perfect, but it is pretty complete, I think.
*Here are the Spanish Equivilants to all the names I've listed here.
English-- Spanish (pronounciation)
Anna--Ana {ahh-nuh}
Antoinette-- Antonieta {an (like ant)-toe-nee-et (like in get)-uh}
Catherine-- Catalina {Cah-tah-lee-nuh}
Isabella--Isabella {ee-suh-bay-ya} (double l is prounounced like a y)
Isabella--Isabela {ee-suh-bay-luh}
Isabelle--Isabel {ee-suh-bell}
Joan-- I have absolutely no idea. There is a Catalan name Joan, but that is usually a male name. There is also a Catalan and Portugese name, Joana, but I've never met anyone named that. It would be pronounced yo-aah-nuh
Josepha--Josepha {ho-say-fah}
Margaret--Margarita (like the drink) {mar-gah-ree-tah}
Maria--Maria {mah-ree-ah}
Marianne--Mariana {mah-ree-ah-nah}
Marie--Maria
Mary--Maria
Louise- Luisa {Lou-ee-sah}
Beatrice--Beatrix {Bay-ah-trees}
Beatrice--Beatrice {Bay-ah-tree-say}
Charlotte-- Carlotta {car-lot-ah}
Eleonor--Eleonor {A-lay-o-nor (nor rhymes with door)}, Leonor {Lay-o-nor (nor rhymes with door)}
Eulalia--Eulalia {you-lay-lee-ah} (also a name of a 4th century saint from Spain, who is the patron saint of Barcelona)
Mercedes--Mercedes {mare-say-days} (it's really a rather common Spanish name)
Theresa--Theresa, Teresa {tay-ray-sah}