It wouldn't serve your work to provide the thesis for you because your own approaches to and understandings of Romanticism will influence your ability to argue a thesis. Any claim I make may be reasonable but difficult for you to convincingly support.
I notice you labeled the question "history," so I'll suggest you look at Romanticism as a historical item. With that in mind, your thesis should say something about its significance, causes, influences, and/or effects in terms of culture, society, politics, and/or religion. If you look at the Wikipedia entry on Romanticism, you'll find mention of literature, art, and philosophy, as well as a list of numerous countries in which Romanticism was a relevant phenomenon. The first step in creating a strong thesis would be to find your focus within these various parts of the various iterations of the movement.
Consider the following two overarching aspects, and explore what interests you the most to get a good start.
1. Romanticism's place in...
(The entire section contains 2 answers and 600 words.)