As early as May, The CW has been cooking up something big for its DC superhero franchises.

Before, it was Arrow and The Flash that constantly crossed over to each other's universes, and when DC Legends of Tomorrow was developed, characters from the first two franchises also lent a boost to the new series. It came naturally for The Flash to pay a visit to Supergirl even though the show was being aired on CBS.

Now that the latter has been taken on by The CW, its president Mark Pedowitz earlier teased fans: ""Just imagine what our superhero crossover episode is going to look like come December," and asked them to stay tuned to the network's biggest event ever.

As early as now, there are several possibilities being pitched about what would spark this four way crossover event.

According to Mark Guggenheim, there will be an external threat that will involve Supergirl coming over to The Flash, then Arrow and later Legends of Tomorrow, reported Moviepilot.

This is an especially smart way to tie in the series with other DCTV shows and establish the connection among the superheroes.

The report speculated that the external force could be Supergirl villain Brainiac or Anti Monitor who would bring the timelines together.

Another theory that is gaining ground is the Flash's Flashpoint arc as well as destruction of the Oculus in Legends of Tomorrow. As a result, there has been severe ripple effects that will be central to building the storylines for both shows.

A report by iDIgitalTimes posits that the Justice Society of America will play a major role in resetting the timeline.

Adding fuel to this theory is the fact that JSA leader Superman will be appearing in the first two episodes of Supergirl's second season.

Arrow himself Stephen Amell has also hinted that the fifth season of the show will focus on going back to the Arrow's roots as a superpowerless superhero, which opens the doors to the possibility that Batman, another member of the JSA who shares the same situation could drop in for a visit.

Anything is possible, in the expanding universe of DCTV and fans are only waiting official word on which of their theories will become a reality.