Dragon Ball FighterZ will have a third season that will contain five new fighters with the first two being Ultra Instinct Goku and Kefla. Considering both are tied to Tournament of Power, it's possible we will see more additions from other universes.
Ultra Instinct Goku's addition to Dragon Ball FighterZ has been known for almost a month now but Bandai Namco and Arc System Works still pulled a plot twist for the players. It turns out that the Saiyan's latest form will not be the first addition in Season 3 since Kefla, a fusion of Kale and Caulifla, will arrive sooner.
Kefla will join the fight on 28 February 2020 with all the quirks of the fused Saiyans. Just like in the anime, Kefla exhibits some traits from the Legendary Saiyan version of Kale as well, appearing more muscular than the two in their base forms. However, her demeanour is not as brutal as Kale's in berserker mode as the trailer below shows Kefla to be taking Caulifla's whimsy traits.
Meanwhile, Goku appears in the complete Ultra Instinct forms in the trailer. He fought Kefla in the incomplete form and had to use the latter for Jiren, at great expense of his energy.
It's currently unknown whether the form will have some sort of energy-depleting mechanic where he has to revert back in order to charge up during a fight. As far as his moves go, they are what DBZ fans could expect - he is moving too fast for their eyes to register but it's hard to imagine the form will be as powerful in-game as it is in the anime. We do have situations where Yamcha beats Jiren senseless, after all, in the name of balance.
Ultra Instinct Goku's release date has not been confirmed yet but the trailer mentions he will come during Spring 2020.