Geoff Johns recent declaration of Arrow and Man of Steel being two seperate Universes leads me to think that they somehow represent a live action multiverse. It's already been speculated that the Flash will present a Crisis storyline sometime in its future. Usually Crisis events involve two or more universes interacting with one another. Crisis on Inifinite Earths, Infinite Crisis, Final Crisis, etc all show different universal versions of characters joining forces.
The idea that the Flash is at the center of it leads me to direct my attention to Crisis on Infinite Earths where the Barry Allen flash (tv version flash) "dies". The story of the Crisis is that various DC canon universes are being destroyed by the Anti-Monitor, the Flash gets kidnapped, supergirl dies etc. The interesting thing about this story is that it was concieved to cut down the amount of DC canon happening at the time into a consistent multiverse. Basically, there was too much random storytelling across various different platforms that made no sense together.... sound familiar? We have Flash and Arrow on CW, Constantine on NBC, Gotham on Fox, Supergirl on CBS, Batman and Supes on the big screen. All of these appear in their own individual universes, completely unconnected to one another. My theory is that this is entirely on purpose to completely seperate them from one another in the eyes of the viewing audience. WBTV is esentially going "Look, these are so unrelated that they're on different networks".
A crisis event could theoretically see all of them interact and maybe even cross over. An event like this could also find a way to integrate basically every DC movie and TV show ever made (like Smallville) and present a serious cinematic threat with these fan beloved worlds destroyed or at risk. Imagine for a moment, a Jusice League film which starts with Val Kilmers Batman watching his universe basically wiped from existance and then cutting over to say Brandon Rouths Superman seeing the sun just dissappear before doing so himself. You could even have Ryan Reynolds suited up as Green Lantern trying to hold back the storm and failing with all the other Lantern Corps. The threat would be very clear from the get go and fans of particular canon will feel compelled to root for the new characters.
Another good possibility for this is that it would allow DC to mix up canon and provide storylines to characters that have not actually experienced those in each particular circumstances. For example, the Dark Knight could become partial canon to Batfleck by combining both universes into one (which has happened before in the comics). For WB this would be a very advantageous thing for them because in the case of say, one actor quitting a role (see Rober Downey Junior uncertainty), a crisis event could be used to explain the replacement of particular actors. As well as this, Universes merging/breaking apart would give WB all the clout they need to advertise to whatever fans of whichever movies that want. Lets say people only enjoyed Batman Begins, WB could come out and literally say, well the crossover made Battfleck have those experiences so hes basically that guy as well as who he was before... Battfleck is a sequel to say... Batman Begins and Batman Returns. From a marketing standpoint this means that WB can also provide box sets for the movie universe which includes these unrelated movies as perfectly logical canon.
Such crossovers also allow characters such as Power Girl, Alexander Luthor, Ultraman and a miriade of other characters to be considered. Characters that without universal crossovers dont really make any sense.
Peace.