A consideration perhaps, but if (butterflies) Catherine's elder sister Joanna of Beira survives, I bet she would be married to a non-spanish prince.
I agree. Philip IV's pride would not allow him to ever recognize Portugal's independence, let alone legitimize their reign by marrying a Braganza princess to his heir. Another interesting butterfly could be if Teodosio, the Prince of Brazil survives. He was seen as very promising and died suddenly, leaving his ten-year-old half-paralyzed, unstable brother, Afonso, as heir.
As for La Grande Mademoiselle, the Franco-Spanish War (1635-1659) was still on-going. I can't imagine the survival of the Prince of Asturias wrapping the conflict up any sooner. Also Anne of Austria and Richelieu were not fond of Gaston so I can't see them honoring his daughter by making her queen of Spain.
Eleonora Gonzaga (born 1630) would be a good choice for Spain. OTL her family was allied to the Austrian Habsburgs by her marriage to Ferdinand III and her brother's marriage to Isabella Clara of Austria, one of Ferdinand's Tyrolean cousins and sister to his late wife Maria Leopolodine. Also Ferdinand III's stepmother, another Eleonora Gonzaga, was our Eleonora Gonzaga's aunt and encouraged the match. Her family's connections helped make up for her slightly lower rank. However I don't know if the Spanish Habsburgs would value the family ties as much as the Austrian Habsburgs did. She might be still regarded as too lowly. Alternately she could be regarded as a proxy for the Austrian Habsburgs and suitable as a way of strengthening the relationship between the Habsburg branches.
Otherwise Isabella Clara of Austria or Maria Leopolodine of Austria are also possibilities as things get moved around.
As for Italy, there are surprisingly few other choices I can think of. Savoy was firmly an adherent of France, the Medici didn't have any available princesses. The only daughter of the duke of Parma (born 1637) is a little too young and became a nun otl. Maybe one of the d'Este princesses would work, but there's a gap with the closest in age to Balthasar being Anna Beatrice (born 1626) and Isabella (born 1635).
In Germany, there aren't many more to add. There are no princesses in Bavaria. I can't see the Palatinate being a serious candidate with their Protestantism and loss of prestige in the thirty years war. Few German states have the religion and status to be considered 'worthy' of Spain at this point.