Do you have other interests that can somehow be tied to 19th Century French culture and politics? For example, writing about the impact of fairy tales in 19th century French culture, the progression of political sentiment in a specific area of France during that time, food culture in Southern France, cabaret culture and history in Paris, etc? I think it would be much easier to help provide possible ideas and avenues for research if you gave some examples of what your other interests are.

This response is kind of boiler plate and not meant to be condescending: Your reading will be a much more suitable source for generating your topic than this forum. I have no doubt someone here could give you an overview of the historiography of 19th century French culture/politics and some of its blindspots, but I don't think that would be the ideal.

As you're reading, what questions are popping up in your mind? When you try to answer them are you finding entire forests have been sacrificed or are you left with nothing to go off of? The authorities you're reading may well tell you in the footnotes what knowledge is lacking. Is that knowledge something you're burning to have?

The other issue with an outside recommendation is that you'll be spending a ton of time on this thing. It should be something you are happy to become immersed in. That could happen if someone here recommends a topic, but you might also find that you're forcing yourself to chew on reading when your dream topic is in that other book you just put down.

So my recommendation is to just read more. I can't help with 19C France. Apologies if this is unhelpful.

You could write about the restoration, maybe the French intervention in Spain to put an and to the "trienio liberal" and restore Ferdinand VII to power, and how the intervention was seen by the French population.

All in all you should primarily write about something you are interested in.