Almost every area of history has some aspect of it that has not been explored! What are you otherwise interested in? Some emerging fields of study these days:

• Gender history is still popular, not just the simple stuff like "hey maybe women actually did some things in history and maybe we can talk about it" but like, what gender means to a society, what makes a man a man? A woman a woman? What about alternative genders? Masculinities studies has been growing since the 90s or so but it is still a pretty darn quiet area of historical study. I would consider my topic in this field.
• Disability history, what does it mean to be able or disabled in a society? How did that society approach disability? This was a TT topic recently, for some ideas.
• Body history - this one is sort of a broad, overarching category, covering many other fields like gender, disability vs ability, sex, medicine, etc. This has been hot since like 2000 or so. Search for body history in your library database of choice, because most of the discussion is on the academic writing side. This one might be a bit tough for an undergrad as it's newer and nebulous, but it would impress your professor I think!
• Digital history/digital humanities - if you can figure out what this phrase means, come back and tell us please... But really, writing about how to use textmining/datamining, working with GIS data, anything like that, as a means of doing history, super hot right now.

There are archives of materials, particularly from recent centuries, that haven't been examined by historians. One of my colleagues is reading colonial records from the Caribbean that no other historian has ever published anything about. Writing a thesis on an archive like that would be pretty cool, but there are a few considerations to keep in mind.

1. How much time do you have to gather your sources? If you're planning a project now that you'll research and write next fall / spring, you have time to travel to an archive and gather material that might be difficult to access. But if your thesis needs to be done by May, you'll want to find sources that are either geographically close to you, or else already published.

2. What languages can you read? This will, obviously, limit you.

3. Do you have experience reading old handwriting (say, were you lucky enough to learn some paleography)? You might want to stick to things that were typed, or written recently, if you don't have the time or experience to learn, for example, medieval latin shorthand.

4. Do you have funding / money / opportunity to travel? If not, you'll want to stick to things your library can interlibrary loan, or archives within easy reach of the place you live.

A further consideration is your own personal interests - if you're writing a thesis, you want to be working on something that you feel excited about, not something that bores you. There might be a great collection of WWII letters in your local historical society, but that won't be much help if WWII puts you to sleep.

If you have time, a good way to proceed is to read some of the seminal books in your field, and look closely at the things they mention but don't talk about much (ie, did a book mention a medieval saint's life that sounded really cool, but not go into detail?). Then look those things up - see if other historians are also ignoring them (this will involve a lot of research). Often, the answer will be yes - there are a lot of sources out there, and the kinds of questions that interest historians are always changing, so some texts become more or less popular, and some get ignored. But you can only figure out which things are being ignored by doing a lot of reading.

If you're less concerned with writing about a specific time and place, the other option is to see if your university library has any special collections that might be useful. Do they have letters from someone who was historically important, old manuscripts, or an unusually good collection of, say, 16th century travel narratives? Has anyone studied this collection yet? Are there local historical societies that might have records (one of the tiny museums near my school has a really interesting collection of documents that would make a great local history project for someone)?

Talk to your librarians to see what resources they have. Talk to your favorite professors whose classes you really liked and ask if they have any leads that you can follow (most professors have dozens of things on the back burner they'll never get around to doing, and be willing to share). Hit up local historical societies. Read books that you love to see what questions they make you want to answer.

And remember /u/qsertorius 's excellent advice: think small. A thesis will go deep into a source, so you don't actually need a big question (you won't explain why the Roman empire fell - but you might explain, say, the significance of Augustine's reinterpretation of a few particular stories from Livy in Book 3 of City of God to explain the sack of Rome in 410. Think narrow / focused / specific).

Good luck! Finding a topic sucks (especially because you feel pressured to get a good one, and fast!), but can also be a lot of fun. The best ideas come from perseverance and lots of open-minded exploration of books' footnotes.

For a bachelor's thesis, I would think small, like an author study or a book study. I would pick a book/author who isn't super popular but lived in a well-researched society. Maybe some random guy from a source-book you have used in class. For example: there is a ton on Roman late antiquity and a ton on early Christianity, but there might not be a ton on an author you liked from your ancient history source book. I'm thinking someone like Orosius or who has good body of work but who isn't already super famous to modern people.

This type of topic is good because you limit yourself to a small amount of primary documents that you can really sink your teeth into while also having a lot of secondary material to use to better understand your topic. This can also be for any time period and your author can be a novelist, historian, poet, lawmaker, letter-writer, essayist, graffiti artist, etc.

Stay away from wars and politics; those seem to be overrepresented. Women's history is undercovered, as is any history that covers non-white populations in majority white areas.

Perhaps a culture with little or no written records or undeciphered language like the Huns.