Any good story consists of two themes.
1) The hero looking badass and heroic.
2) The hero getting countered by the villain, and having to overcome these great obstacles, to become even more of a hero.
As it always is, the role of the DM is to provide a range of challenges, sometimes to highlight a player's strength, and sometimes to pick on their weaknesses. The key always is balance, which is something 5e recognized always comes back to the DM, at the end of the day no one knows their party like they do, so they will always tailor encounters to find the best cinematic fit.
So if a character is all about fire, then you know I'm going to throw in some encounters with tightly grouped people, or flammable things, so they can see everything burn and feel awesome. And you know I'm going to throw in a fire resist/fire immune monster here and there, to make them go "gulp, what now?". That's just good storytelling.