Make Android layout from design files (Adobe XD), use in-app purchases (IAP) with server-side validation, use social network API (Facebook, Google, Twitter, Instagram), edit image and photos on device, implementing video/audio/text-chat support, use REST API of any service, develop Android SDK (library) for your service with easy and well-documented API, use blockchain technologies, use geolocation and Google Maps / OpenStreetMap, use relational like SQLite and non-relational like MongoDB databases, use cloud services (Firebase, AWS), social networks API (Facebook, Google, Twitter, YouTube etc), use Android SDK (Camera, Phone, Contacts, Messaging, Power, Storage, Notifications, Web browser, Sockets).