I don't believe in combos in this game. I call them "set ups" now. You can do this on the platform of Battlefield, for sure, and a few other areas that have platforms, but some might not work, so it's best you test these tricks on any stage with platforms just to be sure. Here is what you do:
When knocked off by neutral air, characters will fall back only at 0-12% damage.
When knocked off by back air, characters will fall back only at 0-6%.
Bair > FF > nair
Bair > FF > uair > u-smash
Bair > FF > uair > grab > d-throw > fair (might be able to pull off a few afterwards)*
Bair > FF > jab x2 > PKF > SH > nair
Bair > FF > jab x2 > PKF > grab > any throw
Nair > FF > jab x2 > PKF > ?
Try these on every stage with platforms in Training Mode to see which ones work and which ones don't. I've tried these on a few, such as Brinstar's right platform and couldn't perform these because Mario would jump out of the animation, whereas on platforms like the one on Battlefield, he falls back and takes the hits. I don't have a Wii U, so I can't test them for you. FF, or fast falling, is important with these, because if you miss that opportunity, which is quite small, then your opponent will be able to move freely. Remember, these aren't all that practical, but you might be able to make some use of them.
*When you come to the up air for the third option, your opponent can move out of reach of your grab. If you can land this one, however, you might be able to put 48% on them, and that's just on Mario. A bigger character might take more.