In 1651, after the birth of the Infanta Margarita Teresa, the queen of Spain, D. Mariana of Austria, was ill and took a long time to recover from the birth (according to the Velasquez bio I'm reading at the moment, Velasquez was supposed to paint her when he returned from Rome, but due to her illness, was given a commission to paint her step-daughter, the future queen of France instead).
So, let's give Mariana a prod, maybe the Spanish doctors do one too many blood lettings, and she dies shortly thereafter. After a suitable period of mourning, Felipe IV has to marry again. Who or where does he marry? La Grande Mademoiselle is often mentioned, but France and Spain are still at war in 1651/1652, so I'm guessing that that's pretty unlikely. The logical candidates (Eleonora Maddalena Gonzaga and Anna de Medici) are both married already, which means we might get a reasonably "foreign" queen for Spain?
So, let's give Mariana a prod, maybe the Spanish doctors do one too many blood lettings, and she dies shortly thereafter. After a suitable period of mourning, Felipe IV has to marry again. Who or where does he marry? La Grande Mademoiselle is often mentioned, but France and Spain are still at war in 1651/1652, so I'm guessing that that's pretty unlikely. The logical candidates (Eleonora Maddalena Gonzaga and Anna de Medici) are both married already, which means we might get a reasonably "foreign" queen for Spain?