There was a pretty good, albeit brief,
discussion of this a while back. I think Alpha Trion made some good points. I think Leopold would probably look to Bavaria of Saxony of a bride but Maria Vittoria of Spain is also a possibility. A Bavarian alliance could help neutralize Bavaria in the event of another war with France, as opposed to OTL's War of Austrian Succession the French may not penetrate much beyond the Rhine. The War of the Polish Succession probably still plays out like OTL so Saxony's support could be gained for recognition of Augustus III in Poland.
I was thinking that if the War of Polish Succession goes the same for Austria in Italy that Maria Theresa might be married to Don Carlos, OTL Charles III. He gains Naples and Sicily and renounces Tuscany (with the Presidi perhaps) and Parma in favor of Austria. ITTL Maria Theresa would be a less prestigious bride and such an alliance was even briefly considered OTL when she was the heiress. So here it makes even more sense to secure peace with Spain and France and recognize the new settlement in Italy. At the time she is ready to marry Austria needs to secure peace with the Bourbons while Lorraine matters little to them so I tend to think a diplomatic marriage to seal a peace treaty would be more likely than a Lorraine match. This works even better if Leopold marries Maria Vittoria as Carlos could renounce his claims to Parma and Tuscany in favor of his sister with the Duchies passing to her second son to form a Habsburg cadet line in central Italy to balance the Bourbon cadet line in the south, so similar to OTL but the Habsburgs retain Parma.
Not to say that I don't think that Lorraine is a possibility but it seems less likely. The Lorraines weren't
that close to the Habsburgs, Francis' grandmother Eleanor would be Maria Theresa's grandfather Leopold I's sister making Francis her second cousin. I believe the infantes of Portugal, as her first cousins, would be the closest paternal relation. So blood relation is no guarantee of a further marriage alliance. And at any rate the current Duchess of Lorraine was of the House of Orleans and IRC angling for a match between her daughter and Louis XV. So there's always the possibility that Lorraine falls into the French orbit as an act of self preservation.
Regardless the political effects would be substantial. No Pragmatic Sanction and all the associated wrangling and concessions that went with it. Maybe they keep the Ostend Co maybe they don't. I think it would also be interesting if Austria manages to avoid the Austro-Turkish War of 35-39, allowing it to retain Serbia and Oltenia. As was pointed out in the other thread this would put Austria in a better position when hostilities inevitable resume in Germany. And long term an Austrian Serbia has some really interesting knock-on effects.