During the game's Kickstarter in late 2017, several characters were recruited and guaranteed to be added to the game as fighters post-release. 6 character were advertised as such, with Bullet Kin and Kick being the first two to get added.
The Leap Day Update was delayed several months due to another studio being unresponsive, according to Trevor. The delay was highly speculated to feature a new fighter, since Yolk was created as a replacement, and this fighter was later revealed to be Meat Boy in December 2020.
MonCraft is a monster tamer / survival hybrid game based on Pokemon and Minecraft being worked on by Lowe Bros alongside Indie Pogo. In July 2020, Trevor commented that once the game is done, or close to, a MonCraft character will be included as a fighter in Indie Pogo.
Also included in the Kickstarter was a list of recruited series that will be represented by stages. 4 stages were advertised as such, with INK being included base game and the Nitrome stage being scrapped in favour of Yolk.
Community member Segamad has publicly stated that he paid Trevor "a lot of money" for a specific character to be included as a fighter. He has dropped several hints towards who it is, but has since left the server and no leads have been made on figuring it out. The current status of "Project Soda" is unknown.
Several studios and developers have given public approval as a result of pitches or other conversations on Twitter. Keep in mind, these games are not confirmed to be included in the game in any way shape or form.