With the exception of 2 Fast 2 Furious, Dominic Toretto has appeared in every Fast and Furious movie, so Vin Diesel returning for Fast and Furious 9 is hardly surprising.
With Elena having been executed by Cipher, Dom is now raising the boy, who he named after his best friend, Brian.
During The Fate of the Furious, Dom learned about his son, who his ex-lover Elena had kept secret from him.
Presumably that means Dom has been living a quiet life so he can properly look after Brian, but you can bet whatever kicks off the events of Fast and Furious 9 will draw him back into action.