Dragon Ball Z: Kakarot will be mostly about the character of the same name, but there will also be other playable fighters. The newest edition of V-Jump has revealed that Vegeta, Piccolo, and Gohan will all be featured as playable characters throughout the story of Kakarot. These are the first additional characters beyond just Goku who have been confirmed to be playable, so there's a chance that we could see even more fighters revealed in the future.
Gematsu and Ryokutya2089 reported on the issue of V-Jump which listed not only these playable characters but also others who will make appearances to support the four that have been revealed so far. Vegeta, Piccolo, and Gohan being playable may come as a slight surprise seeing how the whole game is named after Goku, though if you've followed the stories within Dragon Ball Z throughout its many adaptations, it makes sense that there would be other playable options. Moments such as Gohan training with Piccolo are important for progressing through the story, so one would imagine that the game would have to give players control of a different character at times to tell the story it wants to.
When you're not playing as one of these three newly confirmed characters, they'll also be present at times as support units fighters that'll assist you in battle. Along with these playable/support hybrids, Krillin, Yamcha, Tien, and Chiaotzu will make appearances as support fighters only. These characters will engage with enemies by responding to player actions which will at times include button prompts, according to Gematsu, and can help out in other ways by either pursuing enemies or guarding against attacks.
How far playing as these extra characters and Goku will take players off the rails of the known Dragon Ball Z story remains to be seen, but we at least know that they'll be featured in some stories that weren't included in the anime and manga. Akira Toriyama, the creator of the Dragon Ball universe, commented on this in the past and said new avenues will be explored.
Dragon Ball Z: Kakarot is scheduled to release for the PlayStation 4, Xbox One, and PC platforms in early 2020.