Prussia was not yet recognized as a major power, so the foreign nations would not have interferred- yet. However, there were several problems.
The first one was, that Frederick was a protestant (although IMO he was an atheist). But that was still a question in that days. Especially in Prussia, where you could be everything, but no catholic. And in Austria, which was much more catholic. Anyway, somehow that might have worked.
The next problem was, that Frederick had a problem with his genitals, as after falling ill he was mistreated by a so called "medician". If he could still produce children, was a question. But perhaps Maria Theresia could convince him...
One positive "problem" was, that both were made out of the same wood, like we say here. Both had similar traits. Both were stubborn and both were giving everything for their countries. And both were intelligent. Elisabeth v. Braunschweig-Wolfenbüttel-Bevern, the OTL wife of Frederick, loved him, but she was not very intelligent. Frederick kept her away because of that. He would not have acted this way with Maria.
The next problem was, if they indeed married, would be the question, as both were heirs to their thrones, if both kept to be the rulers independently or if the marriage meant de facto a union of Austria and Prussia, even if only the first male child was to become archduke of Austria and king of Prussia.
So let's assume this really happened. Both would have a nice time at Rheinsberg until 1740 when their fathers died. I think the Austrian War of Succession would have taken place, but this time with Prussia defending Austria. Also Frederick had now the possibilities to make the Austrian Army a Prussian Army. France, Spain, Sweden, Saxony, Bavaria and some other German and Italian minors would have fought against Austria, Prussia, Russia, Britain and some other German and Italian minors.
I guess, Frederick as supreme commander would have gained fame in this war. Not with fights in Silesia and Bohemia, but Italy and Germany. In the end Sweden would likely had lost Pommerania to Prussia, big parts of Saxony to Prussia and parts of Bavaria and Italy to Austria.
The next step, I assume, would be the attempt to reform the HRE. If they tried to do so, a new war would have happened. This time they might have been on the side of France against Russia and Britain. Hannover would have been occupied very fast, he didn't like his English cousin very much, and then Russia would have had some troubles. Courland would become Prussian I guess.
I also think, that Frederick would now try to keep Poland more or less intact. Perhaps after demanding West Prussia as price for that. Either you give it to us or we'll take it and make an agreement with Russia... Either this, or the OTL partition(s).
In Italy the Austrians would have been able to gain more influence, as well as on the Balcan.
At least now the Alliance with France would break. Likely the French troops would have changed the game in the colonies. As no forces were needed in Europe. So Britain had no continental sabre and had harder fights in the colonies. Perhaps even an earlier rebellion.
If there was another war with France, Frederick might indeed had been able to unite Germany...
Adler