blocks that can be used to make games/dungeons
blocks that can be deleted or placed by non builders
Lava blocks that kill people
see through blocks
blocks that can be turned on or off by switches (colour coded)(used to start races or do puzzles)
Spawn point blocks
This would allow games like spleef to be played easily.
If you want to set a spawnpoint, all you need to do is stand in the desired location and enter "/setspawnpoint" into the chat. All people will be forced to spawn at that exact point.