Sure, more fighters are coming. However no matter how many they add some will just never make the cut.
Popeye. Nintendo has a lot of history with Popeye, he was launch title for Famicom. Somewhere along the way Nintendo lost the license to make Popeye games. So despite some history together, he will never join the fight. Shame too, as he could easily be an alternate for Little Mac.
Will from "Illusion of Gaia". Enix, when they were their own company, made a lot of interesting games. Most of which aren't much remembered today. This Zelda-like game is no exception. He had a flute and could take on the form of a boy, a knight, and a being made of blue flames. This could make for an interesting fighter, that will never be in Smash.
Any character from a Rockstar game. GTA, Bully, Manhunt, Red Dead, any of the protagonist could make for great fighters. Sadly Nintendo doesn't have much of a relationship with Rockstar, and likely never will. Seems odd the GTA game haven't made it to Switch, and it is too late for them to be on 3ds. Last time they had a game on a Nintendo console was Bully on the Wii. Guess that makes Jimmy Hopkins the most likely to show up, and that is still an extreme long shot.
Shodan. The antagonist of System Shock 1 and 2. While these games get more love today than they ever did when they came out, they are still rather niche. Also, not sure how you make an evil artificial intelligence into a fighter exactly.
Gordon Freeman. If Valve isn't going to do anything with him, what chance does Nintendo have?