If you’re into graph theory, existence of strongly regular graphs is an interesting topic. You can also get into some spectral analysis and algebraic graph theory with just a basic knowledge of linear algebra. That being said, it is usually best to find a professor you like and work on whatever they’re doing though. The passion tends to follow the work, and you want to collaborate with professionals that you interact with on a regular basis. Good luck! Undergrad research is super fun and rewarding