B reverse thunder trick.

[Mykelism]

Here's something I noticed you can do if you b-reverse a thunder off the ledge.

 

[SafCar]

You can Thunder Reversal in mid-air too, but it requires some tricky timing. I've done some testing with all the characters in the 3DS version and this is what I got for Reversal Down Specials:
Luigi, Yoshi, RosaLuma, Bowser Jr, Donkey Kong, Diddy Kong, Zelda, Sheik, ZSS, Dark/Pit, Pikachu, Duck Hunt, ROB, Ness, Dr. Mario, and Mega Man all can change momentum and reverse direction during their Down B Special when performed in mid-air.
Some notes about this list, though:

• Luigi and Dr. Mario face the other way after the attack is finished. Still some slight momentum change
• Yoshi only changes the direction he faces. His DI halts and goes straight down.
• Ness' DI is a little wonky, but it does change direction
• The easiest ones to Down B Reversal from my experience are Bowser Jr, either Pit, ROB, and Mega Man.
• The hardest ones to Down B Reversal from my experience are Diddy and Zelda, both of which required 25-50% Speed in 3DS Training to pull off
• This is the only way for Donkey Kong to change direction safely in midair aside from his Side B. This makes it vital for any Donkey Kong player to learn as the timing is not the easiest with him.
• There are also 3 other characters that don't change DI, but can reverse direction. They are

1. Robin
2. Charizard
3. Villager, but only his axe. The Plant and Water Can do not reverse at all.

I am unsure how Pikachu can use this tech, but It could make for excellent edgeguards against high recoveries such as Fox, Falco, Rosalina, etc.