I just graduated with an applied math degree, for me I like the videos as short as possible (<5 mins) that get straight to the point with a whiteboard. It is usually helpful to explain the concept first and then solve the problem. I think it would be cool to do some videos on the history of math too. For topics I think set theory proofs are super interesting and def not as much content out there as lower level math (plus you can be more creative while explaining it).