Many years ago I made a text based manager game (for an ice sport called bandy, that is similar to football (or soccer for Americans)). I never came very far and now (12 years since I last looked at the code) I have decided to start over using Qt. One thing I want to do is to be able to have a dynamic league system, where countries that do not have leagues today may start leagues in the future and countries may extend league system if new teams want to join. The problem is now how I should think when creating the data model for the leagues.

With a fixed set of teams per league it is quite easy, but how to think when I want to do it dynamic?

This sounds like a good use case for a database. Qt can interact with database systems. You could use MySQL , SQLite (File database), Postgres or even something like MongoDB (although Qt does not have a class for this, there is http://docs.mongodb.org/ecosystem/drivers/cpp/)

To reach a broader audience you might consider using something like the MEAN stack instead. This would basically be creating a web application using MongoDB, Express, Angular JS, and Node JS. Then people could use this from a web browser. Bootstrap could be used for the styling. Then you could host it on something like Digital Ocean or Heroku.