I like this one....could be fun! OK so suppose Frederick converts to Catholicism to marry MT. He does not renounce the throne of Prussia but agrees that there should be no Austro-Prussian union. He is elected Holy Roman Emperor.
In the War of Austrian Succession it is Austria, Prussia, Britain, Holland, Denmark, Russia and Portugal vs. France, Spain, Saxony-Poland, Sweden, Turkey, Sardinia and Naples. It is an embarassing set back for France.
Not only is MT confirmed as ruler of Austria, she gains lower Bavaria as well. Frederick is confirmed as Holy Roman Emperor. Prussia gains Swedish Pomerania, West Prussia and Mainz. Russia acquires territories in Eastern Poland and Ukraine. France's colonial empire is diminished to the advantage of Britain.
The marriage would be a strange one obviously. Frederick was not up to the task of procreation, yet MT was a lusty woman. Do they slip a strapping aide-de-camp into MT's bed to produce and heir? Was she too prudish for that? Maybe, maybe not. In OTL MT's sister Maria Anna was childless. What if she married Francis of Lorraine? Would there be heirs? Perhaps.
If there is no heir, Prussia passes to a nephew of Frederick. Is there a second War of Austrian Succession with Saxon, Bavarian and Prussian claimants? One would think Vienna would favor the Prussian claimant with a reenactment of the earlier war resulting.
Then we would have a Hohenzollern dominated Central Europe...a Protestant Branch in Prussia and a Catholic Branch in Austria. This development could lead to a Franco-Russian alliance down the road.
Any other thoughts?