Question is, do you sell your Podcasts?
If yes, do you only focus on pushing sales or you have something unique that is helping your subscribers?
If you don't and you're lost of ideas, then I guess it's the right time you take a break, sit down and ask yourself what value you can provide to these subscribers that will really be valuable then you should have the next topic idea. By doing this, it will give you an idea(s) of what next to SELL to them if you do sell to them.
Another way is to go into surveys. Use survey in a bid to help your subscribers and this way, they should respond to your survey and give you head up on what they want...This should give you an insight into what the next topic should be.
I'm sorry, I'm not an English speaking person, so overlook any typo or grammatical errors.