So it's unclear what you mean by 'powerful or influential', but one example that I think clearly meets the definition is Christina, Queen of Sweden (who, among other things, abdicated, converted back to Catholicism, and moved to Rome).

This is a widow, so I don't know if that's what you're going for, but, Katharina Schuetz Zell. She was the widow of Matthew Zell, an important Protestant figure in Strasbourg. On her husband's death, she became fairly active in the religious life of Strasbourg, arguing that she should be one of the key interpreters of her husband's work. There is a wonderful book that includes a good essay about her and translations of many of her writings called Church Mother: The Writings of a Protestant Refirmer in Sixteenth Century Germany. The translator is Elsie McKee.

You might also want to check out the series that book was published in, The Other Voice in Early Modern Europe.