Well, we can make a HL2.net server easily on Creative mode, but for Survival we'd have to find a way.
EDIT: Okay found the details of making a Survival Server. LINK
I think i can screw around in the minecraft.jar and either delete the cave noises or turn them into other sounds.
switching to headcrab sounds = win
reskinning slimes to look like headcrabs = epic win
God, these cave noises are really starting to piss me off. I turned off sounds, and the game is lacking without them, but with them on, it's just cheap scares.
There's a thread on the minecraft forums about a guy trying to recreate The Imperial City from Oblivion in Alpha mode. It's actually coming along very well!
Also, I realized that you can reskin items to make them look different. So, instead of the default look for the axe, you could essentially make a battle axe and it would look the same ingame.