Echo Fighters for characters first appearing in the original Super Smash Bros. for Nintendo 3DS / Wii U.
45. Isabelle
Original Fighter: Villager
Odds: Very High
Isabelle is the mayor's assistant in Animal Crossing: New Leaf. While this character may not seem like much, she's apparently insanely popular in Japan. There are already multiple Amiibo in both figure and card form available for this character.
Chances are Isabelle will just steal the Villagers moveset and eventually sell out of her Amiibo in Nintendo's home country of Japan. All of this will happen while her Amiibo hangs on pegs here in the States. Did you stop at Toys "R" Us before it closed?
54. Medusa
Original Fighter: Palutena
Odds: High
Medusa is one of the main antagonists of the Kid Icarus series and also holds a staff like Palutena. Due to the similarities, it would likely be easy to adapt Palutena's moveset into an Echo Fighter. Additionally, some of Palutena's custom moves from the Wii U version of Smash Bros. could be slapped on Medusa as they were heavily promoted for Pit's goddess in the previous game.
Also don't forget, Sakurai worked on Kid Icarus: Uprising on the 3DS, so he might have some bias for picking this character.
55. Ms. PAC-MAN
Original Fighter: PAC-MAN
Odds: Very High
Like PAC-MAN, Ms. PAC-MAN was, and really still is, an extremely popular classic arcade game. Ms. PAC-MAN seems like such an obvious choice that I really don't know what else to say about her. I'd put money on it.
56. Celica
Original Fighter: Robin
Odds: Medium
Celica is an interesting way to appeal to both fans of classic Fire Emblem entries as well as the modern 3DS remakes. She was the top voted among female heroes in the 2018 Choose Your Legends poll, and Donald insists that Sakurai won’t hesitate to demonstrate the Echo Fighter concept with an Echoes fighter.
She jumps off of Robin as an Echo as like the Ylissean tactician, Celica is capable of expertly wielding both swords and tomes. There is the possibility of her being an independent character via DLC, but it seems more likely that they’d include a hero from the upcoming Three Houses title if they wanted to take that route, as was done with Corrin.
60. Ken
Original Fighter: Ryu
Odds: Very High
Ken is the Street Fighter version of an Echo Fighter in his own series! So it would make sense for him to appear as he always has in Super Smash Bros. Ultimate.
One of the main differences between Ken and Ryu is that Ken has a stronger Shoryuken, while Ryu has a stronger Hadoken. This should be an easy thing to replicate in Smash, just like it has been in countless Street Fighter games.
Evil Ryu is another option, but doesn't seem likely.
61. Squall
Original Fighter: Cloud
Odds: Medium
For the record, much like the Cloud he’d be based off, Squall has appeared on Nintendo platforms with Kingdom Hearts: Chain of Memories (as Leon) and the Theathrythm games. As Final Fantasy 8 did feature a Limit system, they could pay tribute to one of the most common techniques in the game by giving him a faster charge but weaker hits.
His Lion Heart technique is a dead ringer for the Omnislash as a Final Smash as well. But Square doesn’t push Final Fantasy VIII as much as VII (there’s a remake of VII coming shortly after North America hosts the World Cup of soccer), so this slot may go to a FF character who actually debuted on a Nintendo platform - or perhaps Olberic from Octopath Traveler?
63. Jeanne
Original Fighter: Bayonetta
Odds: High
Bayonetta’s close friend and occasional rival would be a natural Echo, and based on the reaction to Bayonetta at this year’s EVO fighting game tournament, a much needed one. Her differences when playable in the Bayonetta games - the Wicked Weave does 50% more damage, she takes extra damage, harder to activate Witch Time - alone would provide easily explainable differences.
With Rodin the arms dealer already confirmed as an Assist Trophy, there aren't many options if Nintendo wants to include Bayonetta Echoes. Jeanne is the best bet, and also adds another playable female to the cast.