As far as DLC goes, we've already seen Piranha Plant added as a playable character for anyone that picked up the game at launch, and in a skit at The Game Awards, we saw Joker (the protagonist from Persona 5) announced as a fighter, too.
These leaked DLC fighters include the following characters from a wide range of video game franchises. According to MondoMega on ResetEra, a Dragon Quest hero, likely Erdrick (the hero of Dragon Quest III) will be the next character. Two of them, Joker from Persona 5 and the Piranha Plant from the Mario series, have already been revealed, but there is very little we now know about the other fighters. Ultimate on Nintendo Switch. Dataminer Jam1Garner discovered three codenames for upcoming Super Smash Bros. These include codenames for Joker from Persona 5 and the Piranha Plant from the Mario Bros. series, one of them, codenamed "Brave" may be Erdrick, the hero of Dragon Quest 3, the "Hero" class in Dragon Quest is known as Yusha in Japan which translates to courageous. There are also theories "Jack" might actually be a reference to Jack Frost - a demon from the Persona series.
In the second half of December, there was a reported "leak" for Super Smash Bros. Joker will come with Super Smash Bros. Written in katakana, Joker and Jack do share the character for "ji", albeit with a different modifier, but that's the only real link I could find between the two names. The user claims that the leaked fighters are Dragon Quest X's Anlucia and Dragon Quest XI's Hero skin will be available along with Ryu from Ninja Gaiden, Steve from Minecraft, and Doomguy from Doom.