I've learned from Product Design interviews that before answering questions like this, it's most important to ask any clarifying questions and then structure your answer logically. The interviewers were there to test and help me, not to catch me out. I was advised to think of "Enterprise" customers. I then took a very consultative approach to this question, telling the interviewers that I would go through a Mutually Exclusive Collectively Exhaustive (MECE) method to identify buckets of customers and then decide where the biggest, unaddressed growth markets were. My answer was to target the "cryptocurrency/blockchain" space, however, just blatantly saying the word "crypto" without any thought behind it does not show your rationale, it just shows that you're impulsive, so be mindful to understand that interviewers are looking to learn about "you" during interviews, not to get answers to questions they already know the answers to.