Start from the paying moment
Describe the customer, the problem and the specific result that makes them reach for a card. For a payroll calculator it might be a correct take-home figure for their state. For a booking tool it might be the first appointment that arrives without a phone call.
Everything in the MVP should either lead to that moment or make it trustworthy. Everything else goes on a written list called later, with the reason it can wait.
The unglamorous parts you still need
Accounts and password resets. Organizations and roles, if more than one person per customer will log in. Billing, invoices and failed payment handling. Transactional email. An internal admin view so you can help a customer without editing the database by hand.
These rarely appear in pitch decks, but leaving them out means launching a product you cannot sell or support. Plan them from the first sprint.
Architecture decisions that are hard to undo
Decide early how customer data is separated, how permissions work and how background jobs run. Changing those after the first hundred customers is expensive. Choosing a mainstream, well-supported stack such as TypeScript, React and PostgreSQL also makes hiring easier later.
What to measure after launch
Track activation, the share of new accounts that reach the paying moment, before you track anything else. If activation is low, more features will not help; a shorter path to the first result will.
We run our own SaaS products, including CostAnswer and Rehabilty, and both taught us that the first ten minutes of a new account matter more than the feature list.
Bringing in a development partner
If you work with an outside team, give them the paying moment and the later list, not only a feature list. Ask how they would cut scope if the budget shrank by a third. Their answer tells you whether they understand the product. More on how we approach this on our SaaS development page.