So my friends and I are making a private modded server with the intent of making two primary changes to the game, making it look better, and making it harder. However, we're also trying to avoid adding, removing, or editing any items in the game. We will be focusing entirely on mods that make the existing content harder through mechanics alone. This brings me to the main question I have, what mods make the game, specifically mobs, harder but does it in a way that feels like it should be part of vanilla? As for making the game look better, we have optifine, Sonic Ether's Unbelievable Shaders, and hardcore darkness. Are there any asthetic mods that would go well with these?
EDIT: I forgot to mention, we also have dynamic lights.