Well, Anne of Brittany was rather interested in wedding her daughter, Claude, to Charles. Louis XII scotched that plan. Also, Isabella of Aragon, duchess of Milan, offered Bona Sforza as a marriage partner for whichever of Ferdinand of Aragon's grandsons inherited the Spanish realms.
However, since this is 1520 and both abovementioned ladies are married, we have:
Isabel of Portugal
Beatriz of Portugal
Renée de France
Mary Tudor, the Younger
Anne of Hungary & Bohemia
Dorothea of Schleswig-Holstein (later of Denmark)
Isabella of Naples
Giulia of Naples
Any of the Navarrese princesses - Anne, Catherine, Quitterie.
An earlier widowed Marguerite d'Angoulême
Anne d'Alençon, Dowager Marquise of Montferrat
And that's leaving any of the German electoral families out.