It would be great if instead of having to build a new app ground up every time we want to add a few features, if we could simply add those features to the exist

It would be great if instead of having to build a new app ground up every time we want to add a few features, if we could simply add those features to the existing application. This would be a lot easier if that application was open source, but having a modular architecture and malleable/extensible design would be more important. Does anyone have thoughts on application that are designed such that features can be added to it easily?