Make something people want, something around which a business case exists, and something you and your team are well suited to do.
All three areas should be iterated on almost in parallel. Although everyone’s situation is different, contemporary business advice is first to identify a desire that people or companies have. Then, demonstrating a customer-validated prototype that satisfies such a desire in their daily lives turns an idea into a project worth investing in.
Second, test whether a viable business case exists: Are customers willing to pay for the value you create? Is this price higher than your cost of creating that value? In the same way that validating customer demand requires a prototype, testing the product’s value relies on building whatever is needed to provide some version of the envisioned experience.
The final step is to check how feasible the actual implementation is. For many standard products and services, it is less about whether it is possible and more about how it can be done. In the case of new technological frontiers or projects spinning off from fundamental research, the entire order might be turned on its head.