You mean a competition for Cup And League winners cup like the Charity Shield and the Supercopa de España.
There will still be issues for the selection of teams that will play against each other because of the level differences of teams in the Cups as compared to the Leagues. There is also the problem of finding proper slots for the matches. Many teams are already overloaded with matches throughout the season (as high as 3-4 matches per day) because of the time period assigned as to be equivalent to a season (28 days).
Many users keep on comparing this game or make suggestions based on happens in real life in a area or region of football that they have known or are currently following. It is likely to be desirable for many players to have the game as close as possible to what happens in real football. This is not possible due to how this game functions and what were the initial targets of Nordeus - many of which seem are still among their current intentions. Many suggestions like yours have been made here in the past, but little to no consideration has been taken or little has been done to satisfy these. And there are also the big questions. Does Nordeus have people assigned to check the forum for suggestions? Are they ready to consider the suggestions? Do/did the employees have enough football knowledge? On what models did they based themselves for the development of this game?
Clearly, this game has been designed based on simplicity but with a strong intention of generating revenues regularly for quite a long period of time. For sure it has a lot of flaws. If you will see my previous posts (among those of last year), you will see that I have described this game as a casino game rather than a proper football management simulation game. In my opinion, if a user intends to spend a decent amount of money specifically for this game, then this game would likely be below expectations, but for 'free', it is an 'acceptable' and fun game.