Breath of the Wild changed Zelda combat in a tremendous way by doing away with almost all permanent weapons and gear. Other than creating a much more tense experience, the decision forces players to weigh each combat scenario and decide whether it's worthwhile or if they even have the right gear to handle the challenge.
But I want more.
Once Link gets access to more powerful weapons on a regular basis, or if you get the Master Sword and undergo the Trials associated with it, combat's edge is taken off just a bit.
Lynels can still kill you with one look, of course, and require some strategy, but you develop a rhythm in how you approach enemies, especially the Hynox. Shoot the eye, slash, slash, slash, run away, repeat.
That's why there should be more enemies and enemies with different strengths and weaknesses. Certain weapon types could be more effective against specific enemies, or there would be more monsters with elemental weaknesses. Given the apparent theme, there could be more of a light-and-dark motif as well, with Link getting the Light Arrows from the start that enemies might have a weakness to.
It wouldn't be a bad thing to see the Darknut make a return either.
Another possibility relates to Malice, the creeping purple mist associated with Calamity Ganon. Malice played a big role in BotW originally, and it looks like it's still going to be important in the sequel. Malice-altered monsters could be another way to change up combat, for example, where a normally easy Bokoblin suddenly becomes a lot tougher or acquires new abilities.
If Malice can change the entire landscape, it could certainly change monsters.