Dragaonbone bandit spawns? 
Honestly, I haven't ever played a game where the difficulty is consistent from start to finish. Ultimately through leveling, abilities, and crafting I feel we always reach a point where encounters become relatively trivial if they're a tank'n'spank.
The only case where difficulty can be increased is by adding encounter mechanics that can be frustrating in a party-based game.
I mean...they're already somewhat implemented in DAI especially on Nightmare. You have dragons that shock water in the fight zone, alphas and enforcers that require melee micromanagement, terror demons that require massive CC to beat at early levels etc...
They could always just straight-up increase damage at the same time I suppose.
I honestly get my difficulty fix at the early stages (try doing Fallow Mire torches on Nightmare + FF at level 8.. holy difficulty... ).
Late game I just look sexy with my fabulously crafted gear and smash **** to a pulp.