2. Supergirl Arrives
In the comics, once Barry fixed history by stopping himself from saving his mother, Flashpoint resulted in a complete reboot of the DC universe known as The New 52. If The Flash is going to emulate this aspect of Flashpoint, it could result in Supergirl becoming part of the Arrowverse.
As Supergirl is moving from CBS to The CW for its sophomore year, it will no doubt crossover with The Flash pretty regularly, after Barry made a guest appearance on the show last season. This might prove problematic, though, as Supergirl takes place on another Earth (one fans are dubbing Earth-4). As such, it will be difficult for the show to be part of the easy crossovers that have become a staple of the Arrowverse.
With the Flashpoint arc, there is a chance for this to be sorted out. We’re not quite sure how a change to the timeline would result in two separate worlds meshing together but, then again, who cares about logic in a superhero show. We just want more crossovers, damn it!