I personally believe that the main competitors in the next 2 / 3 years are going to be Djokovic, Alcaraz, and Medvedev. But I still do not think that Medvedev is on the same level as the other two. Alcaraz is obviously the wonder child. Djokovic is the experienced one. These two are going to play against each other as long as Djokovic does not start performing badly because of his age. It could be 2 years or it could be more. But I think these two are going to be the center of attraction for that time.
Alcaraz is indeed the wonder child of this generation but will he be able to win 20 or more grandslams like Federer, Nadal and Djokovic? Alcaraz (and Djokovic for the next 2 years) will be the men to beat in the future but nowadays you got so many good and young players that stand a chance (Sinner, Ruud, Medvedev). You also still got the likes Tsitsipas, Kyrgios, Thiem, that can maybe also win a grandslam in a good day.