I think that part of the problem is that the game is still in development. A lot of the content that might spice up the gameplay loop simply hasn't been added yet. And if one stares into the juice void for long enough, the juice void will stare back (i.e. if you play a lot of any one game, it will become boring).
However, elite enemies are - allegedly - going to be getting unique AI in the future to make them more of a challenge. Whether or not elite enemies will scale to the player's level as opposed to just monster level is anyone's guess.
To go along with your idea, though, if Fish chooses to add biomes (which he's teased in the past) then perhaps being in certain biomes can have special effects on monsters, like having them scale in level according to the amount of upgrades the player has from their skill upgrades/weapon modifiers/hats, without the change occurring everywhere in the overworld. (This makes certain areas have increased/decreased difficulty that the player can seek out, perhaps with the promise of special rewards.)