Ohhhh I like the idea of cloud for competitive modes. Make a giant skybridge to escape on, then delete like 5+ blocks as you run. They'll have tons of trouble rebuilding it since they can't stand still.
Fog block would be cool. Reduces visibility. Would be awesome making some platform covered in the stuff (you need fire to remove it) that has holes placed around, so when people chase you they can't see, fall through one of the holes into a fiery death ( <3 lava).
The growth blocks sound pretty interesting, but could it not get out of hand?
I don't know how performance intensive stuff like duplicating blocks can be for this game, but I would certainly love to have a mode in the game that allows for customization of all block properties. Set growth rates, etc. Personally I'd love to plant down a single block and watch it explode into a giant tree when it comes in contact with water.