I think things should be improved upon what's already there.
Endermen should be stronger, faster and overall more though of a foe, more "Weeping Angel" like, if you know what I mean.
The new Fungus biome should act like a corruption zone where most things are hostile and might have special loot or other special addition.
Strongholds should have some sort of creepy hostile mob and more to make it feel creepy and abandonded.
I hate to bring this back up, but I want Herobrine in the game, a very rare sight but some creepy stuff happens when he comes.
Seasons and more weather, simple enough seasons speaks for itself and I think it would be cool if there was foggy days, light rains, hurricanes and during summer fire should go back to the way it was in the earlier Minecraft days, where it could spread forever.
I should really learn how to mod this game, there are sooo many things I wish to add.