Nah. DC universe containst more than superheroes and Constantine works ten times better alone than in a shared universe. I like Justice League Dark and new Constantine, but even they are at best when they pretend the rest of DC hardly exist and even they cant compare to Hellblazer. Same thing with Gotham, the only way it might work is if they get freedom to fuck around with canon.
That's what I'm saying though. You can have the occasional throwaway line like "Superman was seen in who-gives-a-fuck-land doing whatever" in Constantine, but for the most part, the show can act like that stuff doesn't exist while dealing with all the magic stuff it wants to. Arrow, likewise, will probably never even talk about the Flash unless it's a team up or something big happens in the Flash show and they want to drum up interest.
One thing about the DCU is that's it's far more, for lack of a better term, compartmentalized than the Marvel U. Characters come together for cross overs and team books and things, but for the most part, everyone sticks to their particular corner of the world. Much different from the MU, where damn near everyone lives in New York, so the characters are constantly bumping into each other. As such, DC could take a similar approach with TV and movies and probably get away with it.
Further, a premiere character like the Flash getting a show opens the door for DC to allow for much more freedom than Marvel currently does. They could limit the embargos down to "No Trinity or their major supporting characters and villains (your Luthors, your Alfreds, etc), and still have tons to work with. Arrow is getting so much mileage out of B and C (sometimes even D) list characters that it's not exactly wanting for anything the Trinity have on lock.
Gotham, of course, would be the only wrench in things. So that should probably stay to itself.