It seems that Shadow of the Erdtree is going to contain more than just new areas to explore and enemies to face. Elden Ring's game director and the face of the Soulsborne franchise Hidetaka Miyazaki revealed that there will be a way to keep the challenge up no matter how over-levelled you are in the base game.
Speaking with Famitsu, Miyazaki said that the DLC will have a similar power-scaling system to that of Sekiro: Shadows Die Twice. In Sekiro, rather than gaining power through levels, you get attack power after beating significant bosses.
If we're to see something similar in Shadow of the Erdtree, it'll keep high-level players from stomping everything in front of them. It will be interesting to see how it works in the Elden Ring expansion, though, as you can use all sorts of attacks in the base game, from magic rocks to simple sword swings.