You know what they should use Echo Characters for? Two things:
1. Significantly different iterations of characters from previous games.
I've been pretty vocal in saying this, but I miss certain versions of characters from previous games. Melee Bowser holds a place in my heart for how monstrous he is, and I have a preference for how Link played in previous Smashes (not that I'm against BotW Link at all). And I do miss Ganondorf having the elbow, high kick, and front-back kick in lieu of his Smashes (though I completely understand why they are changed).
To this end, I think they should use Echo Characters to bring back these significantly different iterations. This would truly make this game more "Ultimate" than it already is by letting us play the way we want with certain characters in a way:
: Dark Bowser, Dry Bowser, or even False Bowser (w/Tanooki Tail) or Bowser Statue can take Bowser's Melee moveset. However, the Melee-type Bowser Echo needs to have the Koopa Klaw instead of the Flying Slam, as the latter fits current Bowser better while the former fits Melee Bowser better.
: Yes, I know this is an annoyance for many people who want Ganondorf to keep the moveset, but Black Shadow should receive the old Ganondorf moveset. Black Shadow can retain the dark version of Raptor Boost instead of Ganondorf's Flame Choke, and he would use the high-kick up smash that Ganondorf lost as well as the elbow forward smash that both Falcon and Ganondorf have lost. And hey, maybe this will lead to Ganondorf getting the magic that people want so much!
: TP Link should come back with the Smash 4 moveset, with OoT Link being his alt. The boomerang would be the Melee version, but TP Link's would aesthetically be the Gale Boomerang without the wind effect.
2. The first step towards having multiple characters from one third-party franchise.
I've been... somewhat against having multiple third-party characters per franchise, as we discussed on
this thread. However, with Echo Fighters, that could change. All they would really need to do is make models, and they can recycle coding and animations from other characters.
: Akuma should be Ryu's echo. Sorry Ken, but Akuma is the most iconic of the Ryu clones at this point.
: Either Shadow or Metal Sonic. I'd personally prefer Metal Sonic since Shadow being Sonic's echo might create another "Ganondorf's moveset" situation where people would ask why he doesn't have Chaos Spear or other unique stuff.
: Jeanne should be Bayonetta's echo. Not much to discuss there.
: Naked Snake/Big Boss. I can't really think of any differences, though.