9. Law Enforcement
There is a theme that has already been expanded upon by the inclusion of the Flash spin-off, in that there is an emphasis on police presence in the over-arching universe. Not really a big deal at first glance, but when you really take a look into the dynamic of Arrow, you see that the police of Starling city is more than just for show. Indeed, they actually created the character of Quentin Lance specifically for the show, and is this readers favorite character on the series. With Flash and it's lad character, Barry Allen, Barry works for the Central City Police Department as a forensic scientist (Or at least he does in the comics). That leaves us with Jim Gordon, the key character for the newly developing Gotham series. Of course, Gordon is a member of the titular city's police force. It shouldn't be too much of a stretch to think of the way's that this could potentially pan out in the grander scheme of things within a shared universe. Police procedural mainstays like Law & Order, or CSI have been working within each other for years now. So that leads us into...