2) Gotham Crossover
This is one of the most obvious ways to bring Batman into the Arroverse. Just do a crossover with the version of the character and his world that is currently on the airwaves.
Though Gotham airs on Fox and is set in a different continuity, that doesn’t necessarily rule any cross-referencing out. After all, both of those things were true of Supergirl (its first season, at least) but it still managed a team-up episode with The Flash. Not to mention Constantine, which aired one season on NBC before Matt Ryan guest starred on Arrow, retroactively placing the show in the Arroverse. As it stands, Gotham is the only DC live-action show around (discounting those based on Vertigo comics like I, Zombie and Lucifer) that is not connected to the The CW-verse in any way. Clearly, something needs to change.
An Arrowverse/Gotham crossover could take either the Constantine or Supergirl route, but we’d most like to see the likes of Green Arrow and The Flash arrive in Gotham City rather than Bruce visit either Star City or Central City (though that could work also).
As well as being a fun event for superhero TV fans, it could also make for an important moment in Gotham’s story arc. Meeting these fully-fledged superheroes could have a profound effect on the teenage Bruce Wayne and put the idea of putting on a costume to fight crime in the back of his head.
The Queen Consolidated logo has even appeared on a building in the Gotham skyline. Come on, it’s practically begging for a crossover!