I've got some interesting ideas:
1. A yellow onion comes in and moves horizontally across the top of the screen. The onion comes by very quickly, rapidly dropping yellow pikmin holding bomb-rocks, that detonate once they hit a surface/player. After the onion passes off the screen, it comes by for another round, going the opposite direction, but moving very slowly, dropping the pikmin at the same rate of speed, while the onion is still sluggishly making its way to the other side.
Basically, the first strike is a wtfouch, hard to avoid but quick to expire, while the second strike is a hovering omen of death, isolating players to stay away from the targetted area as it moves. Also makes for a nice Grab-ko for olimar, seeing as he is unharmed by these.
2. For a 15-30second period of time, a chain of 3 pikmin block the edges of the stage, making no other character besides olimar able to grab the edge (somewhat of an edgehog deal). This also makes the effectiveness of his pikmin chain improve by quite a bit.
3. All action stops, and Olimar picks 3 more pikmin. These turn out to be 3 yellow pikmin with bomb rocks, and are ADDED to his current party. He is able to use these in all his A attacks, and in his pikmin throw. The catch is, they explode (and die) when attacked with (ie, your smash attack also causes the enemy to explode), as well as his throw turning them into bombs. If olimar uses his up B recovery to grab a ledge, he is doomed, as the yellow pikmin cause the chain to explode, killing all pikmin and sending olimar flying in the opposite direction.
This liability motivates the player to use them quickly, as one fall with these in the party could mean a painful death.