How can I come up with a a good thesis about how a character develops over the course of a literary work?
The main strategy you need to follow to arrive at a workable thesis about a literary work is to think through the difference between an argumentative point and a summary. Simply summarizing the main events that occur to a character or even describing the most important change in the character is still description and summary rather than analysis and argument. Thus to create a thesis about the development for a character you need to make a point concerning the reasons for the character's development, or argue about what the author's intention may have been in having the character develop, or analyze the ethical aspects of the character's development. The other thing to keep in mind is that a good thesis must be controversial, something where good arguments can be made on either side. To say, for example, that the protagonists in Jane Austen's novels mature and gain wisdom isn't a good thesis because it is relatively obvious; one could argue either for or against the claim that Anne's initial dismissal of Wentworth in Persuasion was justified by later events.