Yoooo, I love this idea because they could totally frame it as a sequel to Ridley's trailer! This is how I'd set it up...
Zero Suit Samus gets knocked away fighting Ridley, and several other fighters rush in to help after the explosion we saw in the earlier trailer. But they are quickly taken out like Mario and Mega Man before them. Shulk manages to stay alive due to his visions but is eventually overwhelmed. He gets knocked into a wall, and as he struggles to get back on his feet, realizes that he's found a capsule, presumably what Samus and the others were looking for to begin with. He flips a switch and the capsule slowly opens with a hiss, a light fog enveloping the room. Shulk sees Ridley's eyes piercing through the darkness, and he realizes that he's cornered. Ridley readies his tail as Shulk tries to get to his feet, but a pair of metallic legs appear before him. Ridley growls as a female figure stands between him and Shulk, and the fog dissipates revealing his challenger...
KOS-MOS Re-Activates!
But kicking ensues.