Supergirl Deal Was Made Specifically With Flash/Arrow Crossover In Mind
A report has revealed that when writer and producer Greg Berlanti made his deal with DC Entertainment to do Supergirl at CBS, it was in his contract that the character could appear on Arrow and The Flash. So there was definitely some forward thinking on his part on where he envisioned Supergirl and the rest of the DC TV universe.
Doing this would definitely benefit both networks, and drive ratings for each of the shows up. It's a win-win scenario, especially since CBS and Time Warner each own 50% of The CW.
Another possible addition to the Supergirl/Arrow/Flash is the forthcoming live-action Teen Titans drama being developed by TNT, which is also a subsidiary of Time Warner. But that particular adaptation is being shepherded by someone other than Berlanti so i would say it's not as likely to cross over with Greg's trio are not as likely.
The series will center on Kara Zor-El, Superman's cousin, "who was born on the planet Krypton and escaped amid its destruction years ago. Since arriving on Earth, she has been hiding the powers she shares with her famous cousin. But now at age 24, she decides to embrace her superhuman abilities and be the hero she was always meant to be."