Now that all rumors are dead again. And nothing so far has stood up to take its place, who do we think is the likely remaining fighters? I am going to place bets on
Eliwood from fire emblem
Rillaboom from Pokemon.
These aren't my definitive "most likely," but the selections from my top picks that I feel have the best chance.
Reimu or Quote: Both are iconic Japanese indie games. Reimu is bigger in Japan and much more currently relevant while Quote isn't as laser-focused on specific regions and is closer with Nintendo. Reimu would be a light character with disjointed gohei attacks, multiple projectiles, and tools to counter-zone. Quote would be a Mega Man-style gunner who primarily uses the Polar Star and the Blade in his normals, while his more esoteric/powerful guns would fill out his specials.
Gordon Freeman or Heavy: Would be very out of nowhere, but both their games are very iconic, and would give representation to the shooter genre. Gordon is more iconic and his game more influential, but Heavy would fit into the artstyle of Smash better. Gordon would utilize his crowbar for melee attacks and alien technology for his ranged moves. Heavy would be a heavy brawler (I like BrawlFan1's concept of him using Russian martial arts) with similarly powerful-yet-slow ranged attacks as specials.
I'm unsure how likely another first party is, but if they're on the table, I think Bandana Waddle Dee also has a decent chance. (I'd like to say Porky and Isaac, but I doubt them unless there's some way Nintendo can make money off their properties in the near future.)
Honorable mentions that aren't among my list of wanted characters are Arle, Crash, and Euden. And I consider Lloyd something of a wild card; no idea how likely he is.