Having stronger opponents as you level up is normal, but with time spent learning skills and acquiring stronger gear will keep you on the same level for most of the time. Adding a power delta that always decreases the damage you do while also increasing incoming damage as the main "difficulty" is half-hearted at best and simply tedious to play against. As an example, Monster Hunter mechanics are simple: kill smaller monsters to get materials for stronger gear to kill greater monsters, repeat to tackle ever increasing threats.