The last thing I'll say about the idea of non-video game characters getting into the Smash series, is that the moment it happens is the moment the series jumps the shark.
There are plenty of characters available to include in Smash before a non-gaming character is included. Smash Bros. is a gaming crossover. Sakurai has already shot down the idea of Goku, Spongebob, Iron Man, and others. I'd take that as close to a "0% chance" as one can get. It's not just a "Anything can happen! He's a troll!" thing, it's that it flies directly against the design philosophy of Smash Ultimate, which is to be the biggest crossover in gaming history.
With two slots left, and with several top-of-the-line series to pull from: Resident Evil, Tekken, Devil May Cry, Tales, Minecraft, Crash Bandicoot, Ace Attorney, Ninja Gaiden, King of Fighters, Mortal Kombat, Monster Hunter...and Nintendo series that could receive newcomers in the future, like Xenoblade, Pokemon, Fire Emblem, ARMS...there is quite simply no reason to entertain the idea as legitimately possible besides the occasional, "Heh, wouldn't it be funny if a manga character made it into Smash? The fans would be seething!"
Not saying that anybody is actually thinking it's going to happen anytime soon, but the logic of adding a non-video game character to a game that is striving toward being the "biggest crossover in gaming history" isn't there. It would be like Mickey Mouse suddenly appearing in Avenger's Endgame for no other reason than, "Oh wow! That's a curveball! Look at how surprised it made everyone!"
It goes against the design of the game, especially when there are still high quality characters and series to pull from.