What I would add
Already in: Inkling, King K.Rool and Incineroar as playable characters, Several Songs returning (Meta Crystal, Sonic Boom and Gritzy Desert, just to name a few), all fighters returning, Dark Meta Knight as an alternate costume and several stages returning, as well as Stage Builder returning.
New additions
1. Lugia as a newcomer (complete with several soundtracks, 2 stages (Alolan Tour and Poke Floats) as well as some new Pokeball Pokémon), but also Giratina as a boss
2. Fawful as a newcomer (complete with several soundtracks and Bowser's Castle as a stage)
3. An option to make Bowser, Donkey Kong, Diddy Kong, King K.Rool and King Dedede talk, and giving Bowser a new up special (Curling Ram)
4. More Echoes (Shadow, Dixie Kong and Dark Bowser, just to name a few)
5. New and returning items (F.L.U.D.D and Fan). Due to this, Mario will use his hammer as his down special
6. New First Party Fighters (Bandanna Dee, Bio-Rex, Viridi and Muddy Mole, with bonus content)
7. Midna as a newcomer (complete with a few new soundtracks)
8. Mr.Resetti returning as an assist trophy, plus


and

returning as Pokeball Pokémon, also returning stages like Yoshi's Woolly World and Rainbow Road
9. Shantae, Ray-Man, Knuckles and Chun Li as new third parties (complete with stages and soundtracks)
10. More bosses (Andross, Rayquaza, Mother Brain and Porky)
That is all I got, and I'm pretty sure that Steve and Adeleine would get in smash