Alright, I have been thinking about several choices that I can present over here. While I can´t tell about the synchronization (I think that pony.tube and cy.tube should be the standard sites for making this thing possible), I have asked myself which episodes/routes could be juicy to watch:

>Related to gen 5 news,as they are related to include more species, perhaps this choice is one of the preludes that opens up the path of Equestria´s enlargement:
Griffon the Brush-Off ----> The Lost Treasure of Griffonstone

>The evolution of a very famous character:
Boast Busters ----> Magic Duel -----> No Second Prances

>The comparison between the actual finale and the possible one that could have occurred in 2013:
Magical Mystery Cure vs The Last Problem

>Episodes that are seen as low points:
Owl´s That Ends Well, Mysterious Mare Do Well, Rainbow Falls...

>Any controversial episode that caused mixed reactions among fans
The Cutie Remark, Daring Don´t, To Where and Back Again...