I'm so new to the game (never played minecraft, either), so I really haven't had a chance to max out and get bored. Honestly, I really like all of your other suggestions. Let me add these, just off the top of my head:
Xonotic/CTF came up in another post, which reminded me... a grappling hook would be nice (maybe we already have one and I just didn't discover it yet?)
I've started poking around the mod code -- it seems like protection offers the option of blocking PvP within its radius, we just have that feature disabled. Maybe add a "super protection" node that enables it? Alternately, there's a feature in the code where protection blocks PvP during daytime, but allows it at night.
Zombies: instead of "brrrraaaaaiiiinnnnns", what if they were attracted to gold, or other shiny things? Same spawn chances, but given the choice, they go after the "shinier" player?
Backpack: doubles your carrying capacity, but cuts your movement rate in half. Also, when in use, all falls greater than 2 blocks deal damage, but the pack adds hit points similar to a shield. Pack should be deployed like armor, using the "sixth spot" on the armor tab: just slightly too difficult to remove quickly.
Pitch-black darkness: The other night, I was stumbling to the kitchen for a midnight snack, in the dark, and realized that IRL you have tactile feedback even when you can't see. Maybe at light level 0, when you bump into things, the light level in front of you jumps to 1 (nearly pitch black) for a fraction of a second, so you can get some teeny bit of information about what you just ran into... or at least know that you did run into something.
How about an upgraded raft? Craft with 5 steel instead of 5 wood, but it can withstand a shark bite or two?
Let me know what you think...