In my opinion Smash Ultimate really could've gone further with the Echo Fighters. Richter and Ken are cool, but I'd like more echoes of classic Smash characters we've been playing since the beginning. It's like a new way to play an old favorite. So here are my ideas.
Donkey Kong -- Obvious as a huge DK fan I want another Kong in this game. I don't believe Dixie Kong would work as an echo, so let's forget her for now. Funky Kong is a memorable, reliable NPC who's been there since the original DKC, so he deserves a spot in Smash. Give him all of DK's custom moves from Smash 4 that had windboxes, since he used to be a pilot. In addition slap that surfboard onto some of Funky's animations just for visual effect (his Up+B in particular), and perhaps most importantly, give him his real voice. None of that "realistic animal" junk--Funky's voice is just way too iconic.
Donkey Kong -- I warned ya. I want two Echo Fighters for DK, with the other one being Chunky Kong. Simply give the rest of DK's old custom moves to Chunky and make him slightly slower. That's all I want out of him. He's an incredibly underused character, only appearing in two games ever, so this would be a nice treat. The memes would be great too since he's the "One Hell of a Guy" the DK Rap talks about.
Young Link -- Ravio. LINK BETWEEN WORLDS SPOILERS AHEAD. Ravio is literally just Link from another dimension and he sells you all of the items you use in the game, so pretty much everything in Young Link's moveset would work for him. Zelda fans might complain that he's yet another insubstantial Zelda roster addition...but I don't care. I like him a lot.
Fox -- Slippy Toad. I think Slippy deserves a spot in Smash with how iconic he is to the series. He even had his own game. Yeah, some folks might complain about "a third Fox clone", but at the end of the day I don't think these guys would fight all that differently anyway. Slippy could be like Fox, but slower and maybe more slippery since he's covered in mucus at all times. Replace the Firefox with something a little goofier but functionally identical and, for heck's sake, give him some dope ass taunts.
Those are pretty much the only ones I want. Let's get creative here!