By the time you get to this stage, you'll know exactly which features your product needs, what works and what doesn't.
That said, It is extremely important not to be infected by 'feature creep' syndrome when building an MVP.
To help mitigate this problem, use a Strategic IO map that will help you avoid unnecessary features so that you can minimise financial and time expenditure.
Here's a link that shows you how to put a strategic Io map together:
http://www.dbrmfg.co.nz/Strategy%20Constraint%20Management%20Model.htmYou need to set your "Goal" as the problem that you're solving in the market.
e.g if you're building an app that helps learn a new language, then the goal would be "speak new language" and then below that would be the necessary conditions and critical success factors that are needed for that to happen.
From there you can create a minimal spec of what your MVP needs and you can be certain that it will viable enough.
Always build in ways to make sure you collect plenty of data though. I personally like to use Mixpanel and Google Analytics for collecting data.