Do you truly understand the customer need? What is the problem you're trying to solve?
How will you make money? (What is the monetisation model and business plan?)
How will you protect your idea?
Who are your competitors and what makes you better?
What are the essential features needed to properly test the idea? This is called the Minimum Viable Proposition (MVP) and you need to make sure you also know the success criteria metrics to test/ learn/ pivot.
Does you app have a great design? Does the landing screen showcase the main purpose of the app without the need of explanation? This is called the User Interface/ Experience ("UI/ UX") and is probably the single most important success factor.
Do you have enough money to give the idea a fair chance to succeed? Not just to build it, but to market, maintain and improve it.
Here comes the techie bit. What language/s or platform/s code will your app and backend systems be built in or on? Native, hybrid, PWA or a responsive site? There are pros and cons of each. What coding standards will be used, who will own the code?
Who is responsible for testing and quality assurance? Which operating systems, devices and browsers will it be tested on?
What security and data protection practices are in place to comply with POPI, GDPR etc?
Will you, as the business owner, get the reporting and admin functions you need to manage and grow the business?
How will your app actually get published on the app stores? How will you receive payments?
Do you understand the app approval, signing and publication process and requirements? (If an app isn't compliant it can delay projects by months).
How will you make sure your app appears prominently on the app stores to maximise downloads (screenshots, descriptions, keywords etc.)? (This is called App Store Optimisation or ASO).
Does your app require marketing and advertising, including a website, social media integration, award nomination and publicity?
Who is going to provide support, maintenance and hosting? What is the approach to implementing enhancements?