There only one more prediction behind that mega Electro Ball accept those two-
-When Zekrem took Pikachu's Power & give it back later , He also gave Pikachu some extra Power or ability!
But then again , Why didn't Pikachu use that extra power until now!
So , This can't happen.
Although , I did thought about the possibility of Pikachu learning Fusion Bolt because of Zekrem and then we got a movie/Game event where we can download Ash's Pikachu with that move.