So, when Margarethe of Austria died in 1611. There were those who proposed that Felipe III marry again. Two candidates were the sisters Margherita (b. 1591) and Eleonora Gonzaga (b.1598).
What if (for whatever reason) Felipe III takes the plunge and marries again? For one, I can see the War of the Mantovan Succession looking a lot more interesting esp. if the new Gonzaga queen has children.