You might know the term Minimum Viable Product, or MVP, popularized by Eric Ries. Hardly ever does someone define a Maximum Viable Product, or MaVP. Products are not complete when there is no feature left to added. Products are complete when there is no feature left to be removed (inspired by Antoine de Saint-Exupery). An API can help you shape your MaVP.
A Maximum Viable Product is a product with the maximum amount of features it could have to support the user in achieving a certain goal, yet stopping short of having too many and confusing the user with what he has to do with the product to achieve the goal. It removes the false promise that more features will increase usage and thus product-love. However, more features doesn't always convert into a better overall user experience, nor is increased usage an indicator for a better experience.
In order to achieve a MaVP it is important to have a good vision, to be stubborn, but it is equally as important to have an API. An API enables you to focus on the MaVP, while you provide others with the opportunity to extend your product in different ways, without falling for the feature trap (adding endless amounts of features and configuration options). Your product will have a consistent and most likely great user experience, while others, who are extending your product via the API, are able to create a great user experience in their solution as well.
As soon as you start to build an option or configuration screen, think if it is really necessary to have it all. What you might need is an overview of what third parties have done with your API and what implementations you are endorsing. You might even want to include an appmarket in which people can find plugins or apps for your product, which can be included directly in your product to enrich the experience. However, even that might be just too much for your MaVP, since it could dilute the overall user experience.
Therefore keep focused on your product, remove features when you can and make sure your API is in sync with the functionality offered in your product. You can create the user experience, however you need the API in the end to make things really scale and to increase user adoption.