This is a great question to think about. Overtly, these two excellent short stories have little in common, but if I were you, I would try to think through comparing how dreams and the imagination are used in both to liberate and free the central characters. It is clear that Walter Mitty finds a sense of release and freedom through his dreams from his humdrum life and banal existence, and above all, from his overbearing wife. He is an oppressed person in so many ways, and his dreams allow him to believe he is someone else and to enjoy power, confidence, prestige and liberty for all-too fleeting moments. In the same way, Mrs. Mallard for one precious hour is able to dream of liberty and release from the state of marriage, and the kind of life that she can lead now that she is not beholden to anyone and can be independent. Both characters use dreams to explore freedom, and both sets of dreams are only temporary before reality comes crushing back down upon them, with tragic consequences in the case of Mrs. Mallard. Thus a thesis statement you could use would be:
"The Secret Life of Walter Mitty" and "The Story of an Hour" both use dreams and the imagination to allow their central characters to experience freedom and liberty.
This would allow you to explore the function of dreams and the imagination in both stories and the temporary nature of the freedom that the central characters obtain. Good luck!