There would be a global ELO system for each user.
After one of the contests end, the backend would calculate delta elo and apply it.
Perhaps also adding a simple "category" system would not be too hard (e.g. 0-1000 -> Novice, 1000-1500 -> Ambitious, 1500-2000 -> Well-trained, ..., 3500- 4000 -> Master, etc.).
There would be a global ELO system for each user. After one of the contests end, the backend would calculate delta elo and apply it.
Perhaps also adding a simple "category" system would not be too hard (e.g. 0-1000 -> Novice, 1000-1500 -> Ambitious, 1500-2000 -> Well-trained, ..., 3500- 4000 -> Master, etc.).