Adding new enemies is only one aspect, but its not enough on its own. You can add as many varied enemies with new mechanics to them as you want, but as long as the fundamental core of the combat is shallow as it is now, all that work for extra monsters won't matter much beyond 5 seconds when you first encounter them and understand how they work. Once the player has figured out what he is dealing with, the excitment of a new enemy type vanishes as fast as it came and turns into a routine, which is ultimately the same old grindy and boring combat it was before
I completely disagree, based on my experiences. Just the other day, I was going into the lava flats for the first time ever, and I started encountering the utahraptors, which can jump to get you, sometimes several times. So, according to your theory, once I figured out how to fight them, it would be a cakewalk? Not so. You couldn't always count on being able to jump over them, terrain dictated if you could get the height to clear them or not, and if you didn't plan a route to move before fighting them, it would be fruitless (this was in a windstorm btw). And indoors? Don't get me started. The exact placement of each raptor in the room made each room compeltely different. If the raptor was at the end of a hallway with a door behind it, I would need to duck into a door between now and when I reached them, and just ignore them, less they hit me and trap me in a corner. If they were at the top of a stairwell, I would have to engage EXACTLY at the apex of their travel time, or else they would hit and kill my before I defeated them. If they were at the top of a ladder, it was a harrowing amount of baiting to get them to come down but dodge out of the way at the last second to avoid being smooshed under their fall.
Of course, this all only applied because I was on a high difficulty and in a windstorm, so each battle was really a life or death situation. One wrong move and I was hosed. If raptors were just skelebot reskins, it would be boring, but because they could interact with the environment in such a way, it made everything more complex. Not saying more enemies need jumping, just that it didn't matter that I "Learned the pattern" of the utahraptors, it was still very fun and VERY hard!
EDIT: I don't disagree that the majority of the surface-chunk combat is pretty shallow, in that it devolves into finding the best angle of attack and just holding down an attack button while jumping over the lighting balls sliding across the ground. Cave combat is a bit more interesting, because you have to place a large portion of the platforms you are going to be using, sometimes while in combat. Sometimes the cave conspires against you, and there's just no good place to fight a shadow bat. Water combat, is pardon my pun, shallow. It's like cave combat, except your platforms try to escape from you... Thanks, platforms, fat lot of good you'll do me now. Building combat is the most fun and varied to me. Sharp angles, weird corners and hidey holes, and the ability to do room transitions often if needed make it the best. It probably has something to do with most of the rooms being handcrafted...