I thought of a cool way they could have Superman finally show up in the Flash/Arrow verse, and it's basically the way he always makes his debut: Have one or more characters on a plane, maybe fighting a bad guy, and the encounter ends with the plane damaged and plummeting to the ground. There's no way to save it, until a red-blue blur streaks past the window and delivers the plane safely to the ground. You could have GL show up the exact same way of course, and with Hal in particular you'd have the added visual of using his ring to fill in the pieces of the plane that have fallen off while taking the controls and leveling out the descent.