I was looking for princesses lying around in the 1700s that weren't French or from pro-French families, but I don't know how many of them would do for an empress.
1. Francisca Josefa of Portugal (b.1699)
2. Anna Charlotte Amalie of Nassau-Dietz (.1710), OTL Erbprinzessin of Baden-Durlach
3. Auguste Johanna of Baden-Baden (.1706), OTL duchesse d'Orleans (but who's parents were both staunch supporters of the emperor)
4. Theodora of Hesse-Darmstadt (.1706) daughter of the Viceroy of Milan and his Belgian Catholic wife, OTL duchess of Guastalla
5. Polyxene Christine of Hesse-Rheinfels-Rothenburg (.1706), OTL princess of Carignan
6. Wilhelmine Sophie of Hesse-Kassel (.1695)
7. Anne of Hannover (.1709) OTL princess of Orange
8. Amalie of Hannover (.1711) OTL one time fiancee of Friedrich the Great
9. Wilhelmine of Prussia (.1709) OTL Margravine of Bayreuth
10. Henriette of Brandenburg-Schwedt (.1702) OTL Erbprinzessin of Wurttemberg
11. Anna Sophie Charlotte of Brandenburg-Schwedt (.1706) OTL duchess of Saxe-Eisenach
12. Luise Wilhelmine of Brandenburg-Schwedt (.1709)
13. Charlotte Amalie of Denmark (.1706), Josef I had originally been proposed as husband to her aunt Sophie Hedwig (who had also been betrothed abortively to Carl XII and to Johann George III of Saxony)
14. Carolina Sobieska (.1697) OTL duchesse de Bouillon (a match arranged by Karl VI)
15. Clementina Sobieska (.1702) OTL wife of James Francis Edward Stuart
And the only Italian princesses I can find are d'Estes - Amalia Giuseppina (b.1699) and Enrichetta (b.1702), as well as Elisabeth Therese (b.1711) and Anne Charlotte of Lorraine (.1714)
Besides these last four, the Sobieskas, Theodora, Polyxene and Auguste, every one of these would have to convert which they might refuse to.
Likewise Friedrich Wilhelm I of Prussia would more likely marry his cousins, the Schwedt princesses to the Hapsburgs, than his daughter. One of his big fears was that he wasn't one of the elect and would thus burn in Hell, so he might not want to let his daughter betray the protestant religion by marrying a Catholic. Also, he stamped on Friedrich the Great's plan to marry Maria Theresia (a Catholic) and Anna Leopoldovna (an Orthodox)