Task Manager App

I’d like to make a task manager app for smartphones (can be iPhone only to start). Should have simple features like due date, tags, priority. Want to save completed tasks somewhere to show progress. Main feature to differentiate it from others is the ability to easily break a tasks into smaller tasks, and show only lowest level tasks. The idea is if a task is taking too long to complete, you should break it up into manageable sized tasks you’ll actually do. I just need the structure of the app made, so I can polish it up as needed. I can do the graphics myself, just need function right now.