Part of the fun of building something 0-1 is that you have a green field in front of you — you can build anything! (Or that's what we wish as PMS....)
As much fun as it would be to the world is your product oyster, constraints help provide focus and a direction. I see the first step is making sure that you and stakeholders are aligned on what a goal or success looks like. And usually, that comes in the form of a metric that you're trying to move.
If you work for an ecommerce company, you'll make very different decisions around what to build if your metric is increasing ASP versus unlocking a new vertical or segment. If your metric was the former, you might start doing user research on customer willingness to pay, or what they're thinking about when they're in the check-out flow. And if it's the latter, you'd probably start with user research into people who aren't yet your customers.
Without that first constraint — what metric do we want to move, what does success look like — there's no way to focus your research and problem defintion phase. Once you have that north star to work toward, you can enter user research with your specific goal in mind.