Launch. If it's successful that's great and you'll be adding an additional feature to it later on. If not you can still take it out of the store, wait for the new feature to be ready and republish it. BTW: adding a new feature to an app has never changed the download figures of my apps, it can change the user satisfaction or the number of stuff they buy in your app. I know it's a strange thing to say but it's a fact. Changing your marketing (keywords, icon, screenshots) is far more effective