alexeypegov's answer is excellent and I would strongly recommend interviewing your potential user base about what the features they would anticipate in a product like yours. However since users might not always be aware of what features they might find useful, it will also be really useful if you perform some basic usability and feature analysis tests with some low fidelity prototypes to see how your users use the system . Also since you are looking at this product as a Minimum viable product, be careful that you dont miss out on small but related things which might not seem significant in the short run but can ruin the user experience in the long run if they are missed or left (e.g. a password reset or a way to update their email address) .
I recommend reading this article on 37 signals What happens to user experience in a minimum viable product?. To quote the article
Does the minimum-viable approach lead to gaps in the user experience?
It doesn’t have to. There’s a distinction to make: The set of features
you choose to build is one thing. The level you choose to execute at
is another. You can decide whether or not to include a feature like
‘reset password’. But if you decide to do it, you should live up to a
basic standard of execution on the experience side.
Features can be
different sizes with more or less complexity, but quality of
experience should be constant across all features. That constant
quality of experience is what gives your customers trust. It
demonstrates to them that whatever you build, you build well.
I also recommend looking at this slideshare presentation The UX of minimum viable products