How to research what products to expand into?

Hi all, left my finance role last year to start my own company. Started selling a niche consumer product, and due to how small the market is, I have reason to believe that I'm tapping out at my current $10mm / year revenue runrate. How would I go about methodically researching/deciding what ancillary products/markets to pursue? For those of you that have done research into expanding product lines for your portfolio companies, what kind of analysis did you perform / any advice on how to think about this? Thanks in advance.