You might be on the right track in developing this thesis. I think that you might want spend some time exploring the political dimensions put forth in Herbert's work. Remember that your thesis is going to have to prove something. I like the "importance of the Fremen" as a point of launching, but I think more detail and precision will be needed in terms of what it actually going to be proven. It would be good to examine the state of politics in the setting of the work. Addressing questions such as which party has power and how it is used, contrasting these answers with which parties act as resistance and how do they catapult their issues of resistance would be good points of discussion. Additionally, it is going to be a part of your paper to discuss the values of the Fremen in the work and what they do to articulate these ideals throughout the work. I think it would be interesting to do some background work on the composition of the novel in terms of time period and political setting to see if Dune might be trying to pull from actual experience as a point of reference in the novel. These are all angles that I would take in beginning to compose a paper based on this thesis statement.