Mario's side-B

[Goten21]

It can throw back so many things now!
Pika's thunder (if he's above Pika and does it, pika will get hit)
Pika's side-B
Villager's loid rocket

It basically gives them less options to recover

 

[Saxophoneoftime]

I thought it could always reflect the former two since Melee.

 

[Poe]

I thought it could always reflect the former two since Melee.

You are correct.

 

[Raijinken]

It's always been able to reflect Thunder, too, unless I'm terribly misremembering. Also of note, it can reflect about 80% of Megaman's moveset, including his fsmash.

 

[Leonyx]

I noticed while playing the demo (finally got it!) that Mario's cape causes Megaman's Uair to go downward, spiking Megaman. Could be deadly offstage.