KDR: I just don't like the idea of picking up a Metroid. Perhaps if they were in containment jars when they appear into the field, that way you can throw it at people, but once they are free I would want them to be "alive" and act on its own will. I like the idea that its going to seek out the healthest character and attack them first.
You could also have it that if you hurt the Metroid, but don't kill it, it will seek an "easier" target to kill. That way there is alittle more control over the Metroids.
Your Pow Block is an awesome idea. I think I would want the POW to do a Smash attack to everyone on the ground. If you have high damage you better jump to block it...
The Lightning Bolt has Mario Kart history as shrinking people. Nintendo fans will know exactly what the powerup does. So I don't think its that drastic to have it hit everyone, and that Randomizer (I say 30% chance it will giant everyone would be great fun.)
Also the Blue Mario Kart shell should be available to use. Throw it and it seeks out 1st place and hits them with a medium sized radius blast to nail surrounding characters.
I do think some items should only be available in 3-4 player multiplayer mayhem.
Lightning, Blue Turtle Shell, definately only 3-4 player games.