I think it would make more sense to use HD textures for the world?
I think you hit a great balance with what you chose to modernize and what you chose to keep the same. If too much was done it would lose the charm and feel of the original game. The excellent pixel art work on the wall and floor textures really defines the game so keeping it in preserves that.
I really like what you did with the monsters. They feel close to the original pixel art and not out of place.
And some of the touches you added are great. I love the way the water octopus dies for instance