To write this sort of theory/application paper, you need to decide on a theoretical approach and a data set to which to apply the theory. The way to arrive at a thesis is to think about whether a given place or given group of works interests you. For example, if you really like spending time in a place like Missoula, Montana or Miami, Florida, you might choose to look at the literary works associated with that place, and then come up with a thesis concerning the relationship of writers to the Montana landscape. If you enjoy French novels, you might choose novels about Paris as a theme. Once you have your group of books, start thinking about a thesis linking them.
You could create a diachronic thesis, by, for example, examining 5 different poems about London written over a 500 year period and examine how attitudes towards London in literature change over time.
The theoretical approach called "eco-criticism" addresses the relationship of humanity to nature. You might examine works about the natural environment by writers including Annie Dillard, Terry Tempest Williams, Rachel Carson, Edward Abbey, and Thoreau in order to see if their writings reveal gender differences in dealing with nature.