What if the fates of Baltasar Carlos and his betrothed, Mariana of Austria - who after his death married his widowed father - were to be switched. Mariana dies either before leaving Vienna or en route to Spain. Obviously Baltasar Carlos is in need of a wife. The nearest archduchesses are in Tyrol, but IDK if they might be considered for the position of Spanish queen, and then there are a flurry of Italian princesses - particularly in Parma and Modena, plus two in Savoy - but again not sure if they would be considered of anywhere near sufficient rank to marry the future king of Spain.
There was a plan floated to marry Baltasar to Mary, OTL princess of Orange (mother of William III), but it foundered due to parliamentary opposition. So, who might marry Baltasar Carlos?