Just a couple of ideas...
By my understanding of the Echo Fighters, each Echo must copy the moveset directly from the fighter it is echoing. The only differences are stats such as speed, dmg, maybe some differences to hitbox/hurtbox, shield strength, etc.
Some ideas for Echo fighters may be:
- Instead of Metal Mario, I'd say probably Cosmic Mario or Shadow Mario from Super Mario Galaxy and Sunshine, respectively. It seems to me like Metal Mario would be way too heavy, and would mess with the Metal Cap item's effect. Nintendo seems to really like referencing Super Mario Galaxy, with the introduction of Rosalina in Mario Kart and Sm4sh, the Mario Galaxy stage in Sm4sh, and the introduction of the Launch Star item in SmUsh, so I don't think it would be too far-fetched to think Cosmic Mario would also make an appearance. The only problem I can think of is a replacement for Mario's F.L.U.D.D. animation. Perhaps they might just give him a dark version of F.L.U.D.D.
- I think Prince Fluff from Kirby's Epic Yarn would be a good contender for Kirby's Echo Fighter. He's pretty much just a P2 skin for multiplayer, so he would have all the abilities as Kirby, but perhaps he could be lighter or something, since he's made out of yarn. One problem, though, is that there is no Copy ability in Epic Yarn, so if they want to stay true to the canon of the game, they'd have to make a change to his neutral B.
- I'm thinking the Classic Sonic that has been appearing in more recent Sonic games (Sonic Generations, Sonic Forces). I don't know if Classic Sonic has a homing attack in the games, though, so idk.
- There are plenty of electric rodents to echo Pikachu and Pichu, so I don't think Nintendo would have trouble finding any Echo Fighters for them. I think the obvious Echo for Pichu would be Plusle/Minun, as stated by many others, and I think good contenders for Pika's Echo are Raichu, completing the evolution cycle, and Emolga from Gen V, which is the closest I think The Pokemon Company has gotten to a direct copy of Pikachu thus far.
- Galacta Knight from Super Star Ultra. Honestly don't know much about this one, but they look similar, so eh...
- Idk maybe the Virtual Boy lul