But she was an one-year-old girl, and until then Austria had no tradition of female inheritance. Who would be elected Emperor? Could Charles II of Spain, the last male Habsburg, receive the HRE? And as the Habsburgs got the Hungarian territories by marriage, would them be granted to Maria Antonia or any of her aunts?
Charles II would appear to be the male heir if Maria Antonia is ignored. What a quandry to be in though, the choice between an infant girl and a mentally handicapped (and future impotent) child (Charles II was only 9). Maria Antonia was next in line to the Spanish throne after her mother (explained below).
Maria Antonia's mother, Margaret Theresa, survived childbirth, and could probably claim the regency, being the mother of the last Austrian Hapsburg, the wife of the latest Hapsburg Emperor, and grand-daughter of the one previous. She also maintained her position in the line of succession to the Spanish throne, creating the line that almost allowed a peaceful end to the Spanish Succession question (Joseph Ferdinand of Bavaria). iOTL Margaret Theresa died three years later, in 1673, following repeated miscarriages and several children. With Leopold dead I think she may be able to live longer, since she won't be having anymore miscarriages or children.
So Maria Antonia is second in line to the Spanish throne and the last Austrian Hapsburg. I think her blood would maintain her as Queen of Hungary, Queen of Bohemia, and Archduchess of Austria. However, she cannot be the Holy Roman Emperor, for obvious reasons. So, then, what happens?
I would propose that Margaret Theresa marry the most powerful of the German princes, and he become the new HRE, but that might be too simple, and I think the Hapsburg would work to maintain power. The problem of course is that "the Hapsburgs" refers to a mentally handicapped boy-king in Spain and an 18 year old widow. And Louis XIV is lurking about.
I think Louis XIV backs Ferdinand Maria, the Elector of Bavaria, in making a play for the office of HRE. And it just might work. So here we are then, Louis XIV has just backed a successful play for the office of Holy Roman Emperor by his ally Ferdinand Maria, making him Emperor Ferdinand IV Wittelsbach. The Hapsburgs appear to be sinking into decline, the English are allied to the French (or at least pro-French neutrals). The decline seems to about to become terminal when word arrives that the Ottoman Sultan is thinking about paying Vienna a visit.
Wow, this just turned into Louis XIV gets his cake and eats it too.