I got this idea while reading the book Atomic Habits. One chapter talks about something called "temptation bundling" and it is designed to get you to do things you need to do but may lack motivation. For example you could decide that you will only play games or watch Netflix after you exercised, studied etc. This app would let you set goals and define what apps/websites are blocked until you finish your exercise.
While I like the idea a lot, there are severe technical limitations, mainly because on iOS you can really block just websites in Safari, or possibly apps if you create custom VPN which is not an easy thing to do.
So maybe the final product would be Mac app with iOS companion. The Mac app would block app and websites on your Mac and iOS companion would tell it when to stop blocking (e.g. you could query HealthKit and get total number of exercise minutes that day).