7) Flashpoint
When we found out that The Flash was adapting the “Flashpoint” comic book storyline, it suggested a wealth of possibilities. In the comics, Barry’s messing with the timeline resulted in The New 52 – a massive reshuffle of the DC universe. As such, it was hoped that the TV version would result in Supergirl being realigned with Earth-1.
Sadly, the showrunners have confirmed that this isn’t the case and Supergirl will remain in her own world (Earth-3? Earth-4?). Still, that doesn’t mean that Flashpoint can’t result in another hero joining the Arrowverse, right?
Whenever they want to introduce Batman into this world, Flashpoint provides an easy explanation for why he hasn’t turned up before now – because he didn’t exist in the previous version of the timeline. Perhaps the time boom caused by Flashpoint resulted in Bruce Wayne’s parents being killed, which they hadn’t been in the previous timeline?
Flashpoint is a gift for the showrunners with which to fiddle with the universe at their leisure and, though they are making use of that to some degree in The Flash, if they don’t utilize the event to make a monumental change to the Arrowverse, it will really be a big missed opportunity.