The Crucible is chock full of so many interesting and complicated characters and a number of compelling themes that your options for addressing it via a thesis statement abound.
You could examine the issue of who is most responsible for the events that take place in Salem, as represented by the play. For example, one might argue that John Proctor is to blame for the tragic events because he had an illicit and illegal affair with Abigail Williams and because he kept the information she shared with him from the court.
One could also argue that Abigail Williams is responsible for the deaths of innocents because of her hypocrisy and apparent lack of conscience—she was attempting to practice witchcraft in the forest and kill Goody Proctor, but then she began to accuse others, knowing it would result in severe punishments.
You could even argue that Deputy Governor Danforth is responsible for the events because he accepts spectral evidence (which cannot be empirically proven) and because he is more concerned about preserving his authority than he is about preserving the lives of innocent people. One could make a similar argument about Reverend Parris.
In addition, depending on what you've studied, you could make an argument about why Miller changed certain facts about the trials, fictionalizing them to some degree. He made Abigail older than she really was, and he leaves no room for doubt that she is conscious and aware of what she is doing and the havoc she is wreaking, all for her own power.
For example, you could argue that Miller made these changes so that the parallels between Abigail and Senator Joseph McCarthy would be more apparent, more effectively commenting on the figurative witch hunt for Communists and Communist-sympathizers in the 1950s and 60s.
You might even focus, simply, on the character of John Proctor, making some argument about his moral ambiguity and its role in the play. For example, you might argue that John Proctor's moral ambiguity—the fact that he does both morally good and morally bad things—leads to the play's theme: that redemption is always possible and integrity is renewable.