So i decided to try and educate myself a little about ARMS since i never played the game, got a lot of extra time right now for some reason. Just wanted to share some thoughts/guesses on which mechanics from the game are likely to get in Smash in some shape or form* :
Homing punches. All the standard attacks in ARMS seem to work like that so it would make sense. It could be part of a special move (like Sonic or Palu autoreticle). Alternatively, maybe many or all of the character's attacks will automatically curve towards the opponent.
A super meter. Pretty self explanatory. Fill the meter and...
(the meter in ARMS fills up when you hit your opponent so it could easily be the same thing in Smash).
A tether grab and/or command grab. Fighters in ARMS can do it, it's already a thing in Smash so yeah.. this one is pretty likely to happen i think.
Various types of Arms. They could do it the 'Byleth way' (by using specific Arms for specific moves), they could also do it the 'Shulk way' and go crazy with a system to switch Arms mid-battle. The craziest way they could do it imo would be the '8 different characters as alts' idea but each of them would have his/her own pair of arms, not likely but still, it'd be insanely hype. In any case, this could easily be the best part of the moveset.
Other possibilities: Auto turn around (like the Ryu, Ken and Terry), tether recovery,
items, attack attributes.
Like i said, it's just some thoughts and guesses based on the limited knowledge i have so far, feel free to correct me if i'm wrong about something.
*Maybe some or all of this has been discussed a lot already idk, i haven't read the whole thread...