I thought I'd compile a list of potential Echo fighters for Smash Ultimate. There are three confirmed Echo fighters. Lucina for Marth, Dark Pit for Pit, and Daisy for Peach so I won't be including them in this list. This list also doesn't include direct clones like Roy since a clone of clone would be redundant.
Mario
- Paper Mario, Shadow Mario
Luigi
- Mr. L, Waluigi, Doctor Luigi
Bowser
- Dry Bowser, Midbus
Yoshi
- Yarn Yoshi, Birdo, Boshi
Rosalina
- Lubba (SMG2)
Bowser Jr.
-Baby Bowser, Boom Boom (this one's weird since he's already a bunch of characters put together)
Wario
-
Donkey Kong
- Funky Kong
Diddy Kong
- Dixie Kong, DK Jr.
Link
- Twilight Princess Link
Toon Link
- Tetra, Ravio
Zelda
- Hilda, Twilight Princess Zelda, Gardevoir
Sheik
- Impa
Ganondorf
- Ganon, Toon Ganondorf, Demise, Nabooru
Samus
- Dark Samus
ZSS
- Tsareena
Kirby
-Prince Fluff, Gooey
King Dedede
- Bonkers, Darunia
Meta Knight
- Galactic Knight, Bandana Dee, Morpho Knight
Pikachu
- Raichu, Detective Pikachu
Pokemon Trainer
- another trainer with echo pokemon of other characters (a ton of options with this)
Mewtwo
- Shadow Mewtwo, Giygas
Greninja
- Frogadier, Croagunk
Lucario
- Riolu, Alolan Marowak
Jigglypuff
- Igglybuff, Wigglytuff
Fox
- Slippy, Krystal (doubt we'll get another Fox clone)
Ness
- Ninten
Lucas
-Kumatora
Mr. Game and Watch
- Paper Mario
Captain Falcon
- Blood Falcon, Black Shadow, Blaziken
Olimar
- make Alph into an echo, Louie, Brittany
Rob
- Chibi Robo
Ice Climbers
- Bubble Bobble (Weird I Know)
Robin
- Celica, Three Houses character
Ike
- Chrom, Three Houses character, Alfonse
Corrin
- Tiki, Three Houses character
Little Mac
- Bald Bull, Glass Joe, Mike Tyson (lol), Hitmonchan, Narcis Prince
Palutena
- Medusa
Wii Fit Trainer
- Just Dance (Silhouette)
Shulk
- Rex
Villager
- Tom Nook, Isabelle
Duck Hunt
- Amaterasu (bit of a stretch)
Sonic
- Shadow, Classic Sonic, Metal Sonic
Megaman
- Megaman X, Zero, Protoman, Bass, Roll
Pac-man
- Ms Pac-man
Cloud
- Zack Fair, Impa (Hyrule Warriors)
Ryu
- Ken, Akuma, Dark Ryu, Sakura
Bayonetta
- Jeanne
Snake
- Big Boss, Liquid Snake, Raiden
Inkling - Octoling
Ridley - Omega Ridley
There's a few I couldn't think of anyone for so I'd appreciate help finishing it off. I'll also add more suggestions from the comments.