A marriage of Don Carlos to Maria Theresa would be a fascinating POD. If Emperor Charles VI had honored the 1725 Treaty of Vienna, then Don Carlos would have married Maria Theresa and Don Philip (Charles’s younger brother) would have married Maria Anna (Maria Theresa’s younger sister). To appease the great powers in TTL, Don Carlos could have waived his succession rights to Spain, Parma, Piacenza, and Tuscany in favor of Don Philip. In return for his sacrifice, Maria Theresa could have been given the kingdoms of Naples and Sicily upon her marriage to Don Carlos. How the more permanent alliance of Spain and Austria would have affected the rest of 18th century Europe’s history is tough to say. In the Anglo-Spanish War of 1727-1729, with the help of Austria, Spain may have been able to have a successful siege of Gibraltar. A defeat of the British in that war would have brought them in closer alliance with their Treaty of Hanover allies, France, Prussia, and the Netherlands. The war of Polish Succession in TTL would test the alliance of Great Britain and France as I doubt the British (or Dutch) would aid France against Spain/Austria in this scenario. France would have a difficult time beating both Spain and Austria at the same time and Augustus III would remain king of Poland. In a peace treaty, Lorraine would remain in the hands of Francis Stephan, and the aforementioned arrangement in Italy for Don Carlos and Don Philip would be confirmed by the great powers.
What would happen to Austria after the death of Charles VI in TTL would also be interesting. I suspect that with Spain in alliance with Austria, and Great Britain and the Netherlands neutral, France would not challenge Maria Theresa’s succession to her father’s hereditary domains. France would probably try to frustrate Don Carlos’s election to the imperial throne, but in the end would fail to stop him. The attitude of Frederick the Great in this TTL would be interesting. I doubt Frederick would have invaded Silesia in TTL, but if he had, he would have faced an Austria focused on him exclusively. The personal dynamics between Don Carlos and Maria Theresa in a marriage would be fascinating. In OTL, Maria Theresa actually loved Francis Stephen but in TTL, I don’t see romance between Carlos and Maria Theresa. I wonder how that would affect how well the two of them worked together.