At first glace a TV Show about Supergirl never seemed interesting at all (Then DC announced Krypton for Syfy) but at second glance it might be genius. Showrunner Greg Berlanti is forbidden to use the big three in any of his DC tv shows like Superman, Batman or Wonder Woman so it seems he is developing his own "trinity" on the small screen. It has been revealed that even though Supergirl will debut on CBS Greg Berlanti has it written in his contract that she may crossover to The CW's Arrow or Flash. This is because Time Warner (subsidiary of Warner Brothers) owns both The CW and CBS which would allow this type of crossover. Unfortunately they obviously do not own NBC or FOX which would explain why we can never see characters from Gotham or Constantine on Arrow.
Another great thing about this type of crossover would be that the "Arrow-verse" will finally get a "Superman" type character without it actually being Superman. Currently casting is underway for his Supergirl but it should be announced shortly. The question still remains if Titans on TNT will also be allowed to crossover since it to is owned by WB. It is also not even on network television which means it won't be stealing viewership from the other DC shows...beside Krypton. But that show is not being run by Greg Berlnati like these three so it might be s stretch to see Nightwing up there.
