I also had a thought.....if you guys decided to do the extra work and make a "transition" PSA so that Emil could switch between Normal Mode and Ratatosk Mode....wouldn't Samus and Zamus fit the bill better than Ike or Falco?
Using a taunt to switch moves seems a little too cheap, especially since both Emil and Ratatosk's Mystic Artes are so powerful. But since Samus can only change form after using a Final Smash, it makes sense to use her.
For example, convert the Zero Laser into Ain Soph Aur (Not really much to do there except change the size and movement of Samus' blast). When Emil finishes using that move, he's forced to enter Normal Mode to regain his strength, and as a result all of his elemental moves lose their elemental attributes and his eyes revert from red back to green. When he acquires the Smash Ball, his energy is restored and he unleashes Ratatosk Mode during Devil's Hellfire, reactivating the elemental moves.
The only problem I foresee with this is converting Zamus' transformation into an actual Final Smash, but feel free to point anything else out that I have overlooked.