I can think of two names who have / had all the potential but couldn't win a grand slam title:

I love this question, and I can think of MANY names (unless we're just talking currently active players who are "running out of time" or past their prime, which I assume to be the case?) If so, I agree with your two names (Nick Kyrgios + Gael Monfils) and I would ALSO add:

▪︎ Grigor Dimitrov
▪︎ Alexander Zverev
▪︎ Stefanos Tsitsipas
▪︎ Borna Coric

I feel like these guys had/have the game to at least steal 1 grand slam in their careers.

Kyrogios was wasted potential, but I never really saw monfils as a chance to win a grand slam. I feel zverev will get one