You can always make the fight harder in many ways. Boss does more damage, has more life, faster enrage timer... There could also be very difficult mechanics they could possible introduce that require faster reaction time and adaptability.

They shouldn't though, because then the encounters would be much too difficult for players outside the .1%. If they tuned T13 to required perfect dps @i130, the fight would be impossible to beat by anyone outside of the very best raiders. That's not good for the game at all.