They should replace the ice climbers with Kraid and Ridley in their original Metroid size. Their final smash will cause metroid prime to come and mutate them into their Super Metroid size using phazon. While Kraid and Ridley are harrassing the other players in their enormousful state, the mother brain's jar house thing will fall from the sky and shatter upon impact with the stage's floor, at which point she will rise up in her mechanical body thing and super laser blast everyone on the field, collaterally reducing Kraid and Ridley back to their original size. Simultaneously, a giant metroid will swoop down and start eating her brain, ending her siege. If anyone is playing as Samus, the metroid will discharge all of its energy into her, enabling her to use her final smash. However, she only has a short time to use it, as a self-destruct timer will have started. Once it runs out, a large explosion will cover the entire lower part of the stage. Then everything goes back to normal.
Also, Kirby should be able to eat assist trophies to gain their powers.