I feel like a good 50% - 60% of these ideas could just be implemented as an alternative costume for a character rather than as a standalone separate fighter requiring a roster spot and some unique animations or attacks. In fact, I'd love if they were done as alternative costumes since it would sort of allow those characters to exist and be used for those that want to fight as them.

Hilda’s color palette is most likely already in the game. As a Zelda series character, she was almost an all-time favorite for me. A Link Between Worlds is an excellent game despite the overall trash character design (Hilda interestingly notwithstanding). The setup was perfect for Hilda to be the main antagonist/final boss then Nintendo lost its nerve. Had Nintendo gone through with it (even if she repented), I’d probably put Hilda right up there with Midna. So close.

Really hate everything about Shadow being an Echo fighter for Sonic over Knuckles, Tails, or even Metal Sonic.

Squeeeeeeeeeeeeeee if Jeanne makes it in.

I’d rather see Medusa as an original character with a unique moveset, or even half-unique. I’ll take her as an Echo Fighter regardless. She checks off three boxes for me: villain, female, and classic.

Funky Kong seems like such an obvious Echo Fighter, I’d be kind of shocked if he didn’t make it. Once change: Up Special should be a surfboard move. All taunts should be surfboard related. Radical.

Ken is also an obvious choice though I kind of hope for Akuma instead. He may require too many changes so Ken it is.

I'd replace Blood Falcon with Black Shadow as the Captain Falcon clone.  Like I've said several times before, with Ganondorf now using a sword in his Smash Attacks combined with the other changes he's been given since Brawl, he's pretty far removed from his Melee counterpart.  Black Shadow would make the most sense to give Melee Ganondorf fans that slower more powerful version of Captain Falcon they liked.  Plus it makes more sense because he has a larger more muscular build and in the F-Zero games Black Shadow has one of the slowest but most powerful vehicles as well.

I wished you hadn't mentioned Lolo and Lala! I hadn't thought of them being a fighter and they'd be a great, unusual choice, but I agree that the chances of them being in the roster, even as an echo fighter is vanishingly small. One thing that may up their chances is that Satoru Iwata worked on the series. That might make Sakurai give the characters a bit more consideration than they would get otherwise.

Funky Kong, Hilda, Dr. Luigi, Dixie Kong, Louie/Brittany, Isabelle, Medusa, Ms. Pac-Man, and Ken would be my picks for great Echo Fighters.

I feel like this can be stated about all of the echoes, both current inclusions and wish lists.