This was a fun exercise - the interviewer in this case, was great and fostered a wonderful discussion. We did a whiteboarding exercise starting with 'WHY', and then thinking about 'WHO' (i.e. target customer segments), followed by thinking through the needed use-cases, prioritizing the use cases, and then thinking through current alternates for merchants and how the app's MVP should deliver that would significantly enhance the experience. Then I boiled down into specific product features and prioritized them using a framework I proposed. This was a fun exercise!