What ? I suggest to add a "Downgrade league" button somewhere in a challenge that would subtract one league level from your current league (ie : Legend => Gold, and so on).
Why ? Codingame is a platform for competitive programming but also learning, it's interesting to try several algorithm and to restart from scratch. For exemple you may already have a good heuristic-based AI that ranks Gold. If you want to try a Genetic-based AI you will have to fight Gold players with a brand new (and probably bad yet) AI, which is very difficult and almost impossible if you are still a beginner. Allowing to downgrade would solve this problem by manually decreasing league until you reach a relevant league.
That's just a suggestion but it's something I often face.