Greg Berlanti can do pretty much whatever he wants with his stable of DC Comics TV shows Arrow, Supergirl, The Flash, and Legends of Tomorrow. If he wants to have them crossover endlessly it's doubtful anybody would complain. A few months ago we learned he'd be producing a feature film centered on another popular DC Comics hero, Booster Gold, and if you thought there would be the same level of freedom you're sadly mistaken.
Speaking with Vulture, Berlanti revealed his Booster Gold film won't be connected to the larger DCU. So don't expect to see any cameos from Ben Affleck's Batman...
"As of right now we have no connective tissue to those worlds. It'd be a separate thing".
Kind of a bummer because the DCU could use the jolt of humor a buddy team-up movie with Booster Gold would bring. Don't be surprised if something changes down the line, perhaps when the film settles on a director.