This past Thanksgiving saw a batch of DC TV rumors hit the net stating Constantine is going to be a part of DC's Legends of Tomorrow Season 2, as well as The Flash appearing on Supergirl.
Now a couple of days later some things have changed.
According to the originator of the rumors, Bleeding Cool, the Constantine rumor might not hold true as another "source" is going so far as to state there may not even be a Season 2 for DC's Legends of Tomorrow. The thought behind that rumor is the series is said to be incredibly expensive, and they are not sure if they will go ahead with a second season, but instead will go with a totally new series of some kind.
IMO, the DC's Legends of Tomorrow rumors more or less seem as just simply ideas being tossed around, than anything else that would be of merit.
Regarding The Flash and Supergirl, the good news is that the original rumor is still holding true (at this time), with it said the storyline may involve an arc from the DC Comics Supergirl comic book about New Krypton, which saw 100,000 Kryptonians come to Earth after Brainiac freed them from the bottle city of Kandor, with a new planet being built for them. I believe the EP's of Supergirl and Flash/Arrow said initially there wouldn't be a crossover, but it seems that Supergirl's ratings have dipped a bit, so they may want to connect it to The Flash, which is a pretty popular show and character right now.