Starlight Glimmer is the first character in the show to be the main antagonist in both the premier and the finale of a Season.
Look closely at the Cutie Mark on the scroll that Starlight is using, Not only is it StarSwirl's, but it also closely resembles Trixie's, potentially foreshadowing the fan theory that Trixie is related to Starswirl
An alternate reality where Trixie took over Equestria was scripted but never made it to the final product.
The first reality that Twilight is sent to has nods to the popular fanfiction Fallout Equestria. Sombra's army wears armor close to that of The BrotherHood of Steel and Celestia's not only have the armor of the Pegasus Enclave, but Rainbow Dash also has her scar.
Each time Starlight Glimmer has appeared outside of the premire this season, Time or time travel is mentioned.
In this episode she uses time travel to get revenge on Twilight Sparkle