I am surprised no one has mentioned Sebastian Korda. I have been very impressed with his results this year. In my opinion his game is more complete than Zverev. He also has a good team around him.Players almost always reach their prime between 22 and 25. NextGen is a weak generation but i still have some hope for Shapovalov. The generation after NextGen looks even worse to me, so it seems like a lasting problem. Its also BS that players cant win slams at a young age anymore. Tadej Pogacar just won Tour De France two times in a row at 21 and 22, and Big3 won their first slam at 19,20 and 22.
Tennis is probably not attracting enough talents anymore, which i think also the absence of US players confirms. There is probably also something wrong with the coaching. The older players careers are inflated, and when they finally retire it will be a wide open field.