It looks like the events for Dragon Ball Xenoverse begin many years after the Dragon Ball Z timeline ended.
The official Dragon Ball Xenoverse website has been updated and says the story takes place in “Age 850”. This is many years after the DBZ warriors defeated Majin Buu as people who have read or seen the manga/anime series. would already know. The Majin Buu saga took place in “Age 774”.
Dragon Ball Xenoverse has an interesting time travel element to the story where your character meets up with Trunks. Time travelling villains Towa and Mira are going back in time to make the villains of the past stronger than before. This has altered the timeline so the player will have to go back and restore history.
There is a strong indication too that the Dragon Ball Xenoverse timeline also takes place after the events from Dragon Ball GT too. We know that Super Saiyan 4 Goku is in the game as well as Super Android 17.
The Jump Festa in Japan is happening this weekend so hopefully some more details about the game are shared very soon. The game is playable at the event for attendees, so a new gameplay trailer is to be expected.