Ferdinand Wenzel was the elder of two sons born to Leopold I, Holy Roman Emperor, and his wife/niece, Margarita Teresa of Spain. Born in September 1667, he died the next January at the age of less than four months. His younger brother, Johann Leopold, and his youngest sister, both died on the day of (Johann Leopold) or shortly after birth (Maria Anna was two weeks old when she died). His only surviving full sister, Maria Antonia, inherited their mother's rights to the Spanish inheritance of the sickly Carlos II, but died before Carlos. Her only surviving son was tipped as a potential successor to the Spanish empire, but likewise died (not without suspicion of poison) before Carlos.
What if Ferdinand Wenzel had survived? If his sister and mother are anything to go by, the auguries don't look good for a long life (in fact, he might even predecease his dad). Would this change anything in the long run? Especially if he were to leave surviving issue?