Majin 21 is a Goku replacement mod. She’s a 12.38MB download. Mastaklo shared a video highlighting how she’ll look in game. This only changes Goku’s character model. It will still use his voice and character icon in the UI.
Majin 21 is an original character who debuted in Dragon Ball FighterZ. She resembles Majin Buu due to her cells being a part of her makeup. She’s also appeared officially in other Dragon Ball games. For example, you could find her in both Dragon Ball Legends and Dragon Ball Z: Dokkan Battle.
This is one of multiple Dragon Ball Z Kakarot character replacement mods Mastaklo worked on. For example, Bardock and Shallot are two other options to use instead of Goku. There is also the Goose from Untitled Goose Game.
As for officially added characters, Bandai Namco still has Dragon Ball Z Kakarot DLC in the works. Trunks: The Warrior of Hope will add Future Trunks in Summer 2021.
Dragon Ball Z Kakarot is available for the PlayStation 4, Xbox One, and PC. The Dragon Ball Z Kakarot Majin 21 mod can be used with the PC release.