Context: I'm a history undergrad and I am looking for interesting topics to write for my final year project next year, thus the question in the title.
North American indigenous peoples prior to 1400. I am reading 1491 and it's amazing how little we know about them: populations, culture, commerce, etc, etc.
There may have been 100 million people in North America, we really don't know much at all about them.
What are you interested in? There is plenty to write about on a macro scale with cultural methodologies, or you can go micro and write social history of the area you live in. The best projects are ones that you care about and invest yourself in.
Probably for a senior thesis you'll want to keep in relatively safe territory. Something that has easily accessible primary source documents and enough secondary literature to help guide or give you context.