Let's be honest with each other, considering the library of characters that Dragon Ball FighterZ could pull from, we're probably going to be seeing a lot of announcements over the next few months of people who we could have guessed would already be in the game. Seriously, it would behoove Bandai Namco to leave anyone out, because you know there are fans out there dying to play all three of the Androids together and they'll be mad as hell if one of them is left out. So expect a lot of these announcements as we get closer to the game's release.
That being said, Bandai Namco revealed that Trunks, son of Vegeta, will be part of the roster for Dragon Ball FighterZ. Initially, at E3 we saw six fighters in the game: Goku, Vegeta, Gohan, Frieza, Cell, and Majin Buu. A few days ago, Siliconera posted a magazine page featuring an adult version of Trunks as part of the game. Whether this is regular grown-up trunks who grew up with Gohan regularly or time traveling Trunks who just keeps coming back to mess up the timeline is kinda unclear, but I'd go with the later as a safe bet and expect them to add him as a kid and teenager as different costumes.