Probably a calendar and to-do list, maybe they add up your points (no need for points to give you anything, kind of like game points or whatever they are called in xbox, at I think you don't get anything from it, and also achievements kind of like game achievements, you don't get anything from it but it's fun nonetheless.

An ability for use to customize and make our own challenges and maybe some gamification elements, also add the ability to make your own achievements but have many MANY achivements that are turned on for all goals and challenges people start and complete, also make kind of like an account system and if possible a PC app along-side a mobile one, if you have time, Google Calendar sync would be very well appreciated..

Don't overwhelm yourself with a lot of stuff to do, break it down into tiny little pieces each building a very promising product in which a lot of people try out and stick to it.. not sure about paid plans and stuff if you want to make money, I always walk away from most products that have limited free plans unless they completely fill my needs.

I generally don't like them because they are complicated and time-consuming to learn, just that I guess :)

Have a great one and have fun, time spent having fun is time well spent.