I honestly just want new movesets for some fighters, of course new fighters and stages, etc. But what I want most is for the game not to focus so much on the fighting aspect. Yes it is a fighting style game but I think after Brawl each new installment had less to offer in terms of interesting and substantial modes. Let me elaborate.
1.) Make a new Subspace Mode or Adventure Mode that vastly improves upon Emissary. A robust 2D Smash Adventure that spans actual worlds from the in game roster. One that takes full advantage of the IP that only Smash can have. Single player modes haven't been necessarily amazing in any Smash but that can and should change.
2.) Fix Break the Targets. Make a return to the Smash 64 and Melee Era of this mode. Give each fighter their own visually and topographically unique stage that return to teaching you how to utilize a fighter while also being a fun exercise.
3.) Bring back Board the Platforms & Race to the Finish. These were fun, mini obstacle courses that only added to the variety of the titles they appeared in. Of course there is room for improvement for these modes but I think they are worth bringing back. New modes that don't focus solely on fighting would be appreciated.
4.) Spirits are cool but...bring back trophies,
I think once HAL stopped being involved with Smash, the fun side modes that incorporated different game genres ceased. I think Smash is in good hands in terms of the main fighting modes. But I think for the next game, whoever is in charge of development should channel the spirit of the first 3 games while of course adding their own flair to it. I just want Smash to have a better variety of modes in the next entry. The roster will probably take a huge cut but I hope 3rd party juggernauts like Simon, Sephiroth, Hero, Joker, etc. stay in.