You can start a podcast about lots of things. Many people start hobby podcasts in fields they have an interest in, and others start podcasts about whatever their expertise might be. I recommend writing down a list of items that you could potentially hold a conversation about and enjoy speaking to others with on your show, and then narrowing down the field to the one general topic that you like. That being said, don't go too general, because the more niche and narrow you go, the easier it is to compete, and the more likely you are to rise and become a leader in a space.