What is a good thesis statement for "The Fall of the House of Usher" as a Gothic story?
There are two points to consider in addressing your question about a thesis for "The Fall of the House of Usher." The first is that a thesis, by definition, must be arguable, which means that the claim, issue or question you assert must give rise to debate, doubt or challenge. In other words, a thesis may not simply claim what is already accepted in the academic community (which in undergraduate studies may sometimes be defined as your classroom....).
The second point is the defining characteristics of Gothic. Some of the defining characteristics of Gothic literature are death, old, crumbling buildings in American Gothic and castles in European and English Gothic, distressed heroines, overpowering villainous men, etc. Therefore your arguable claim, issue or question must relate to some point relevant to Gothic as defined, which might include the value of Gothic, the realism of Gothic, etc.
Based on the above, I can make a couple of suggestions as to what an appropriate thesis might be. For instance, you might assert that even though Usher is not painted by Poe as a villain, such as the villain in The Romance of the Forest by Ann Radcliffe, Usher is nonetheless the Gothic hero of "The Fall of Usher." Another example might be that you could pose a question relating to the literary value of Poe's writing's in the American Gothic genre, as in: "Considering the pristine vividness of Poe's descriptions of horrors in his stories in the American Gothic genre, as illustrated in "The Fall of the House of Usher," do Poe's Gothic horror stories carry any legitimate literary value?"
An additional example might be to examine the realism in American Gothic genre. A related thesis might be, "Even though the horror of "The Fall of the House of Usher" might make Poe's style seem to be antithetical to realism, one of Poe's foremost literary techniques is, in fact, that of realism." Each of these suggested examples presents an idea that is either open to debate, open to doubt, or open to challenge. Further, each presents an idea that does not yet have a definitive answer within the broader academic community.
As with all thesis statements you need to make sure you have certain elements in place. For one thing, you need to understand that you're not just describing a certain feature of the story; you're actually making an argument. And as with all arguments you need to be able to back it up with evidence from the text that you're discussing.
In the case of "The Fall of the House of Usher" you first of all need to define what is meant by "Gothic." Then, once you've done that, you can carefully examine the story to see what elements, if any, could reasonably be described in such terms. One possible approach could be to highlight the supernatural in the story (undoubtedly a Gothic element) and argue whether or not Poe displays any skepticism towards it. So one possible thesis statement could be something along the lines of "Although there are numerous supernatural elements in 'The Fall of the House of Usher' Poe still retains his skepticism concerning the reality of the supernatural." This will allow a meaningful debate to ensue. Does Poe really believe in the existence of the supernatural or is he simply using it to tell a good story, or possibly to convey some deeper meaning about the human soul?