First of all, if Smash 4 gets ported to the Switch, there won't be any character or moveset changes. Sure, there will hopefully be a balance patch, but they're not going to go and give characters entirely new moves. Mario won't get Melee Down B back, Link won't get his old Boomerang back, etc. However, this is what I would like to see.
1. New characters. I have a strong feeling that, if Smash 4 Switch becomes a thing, we're going to be getting some new characters to go along with it, because the best way to get somebody who already has the game to buy a whole new copy of Smash 4 is to put in a couple of new characters, let's be honest. Whether or not they would be version exclusive depends on just exactly how much control Sakurai has over this game (if he's even working on the port to begin with). If Sakurai is heading the port, I could honestly see him wanting to put the new characters in all of the games, due to his insistence upon keeping the rosters the same between all versions of Smash 4. However, if Sakurai isn't heading the port or Nintendo forces Sakurai to do it, then the new characters will be Switch exclusive. I don't think we'd get a lot of characters, though. I'd guess probably like 2-3. Definitely not more than 3, though. Ideally, I'd like to have three new characters. The first would be a returning veteran, the Ice Climbers. Sakurai said they were complete for the Wii U version, so it wouldn't be that much trouble to take the completed Ice Climbers, maybe do some polish and balancing with them, and put them in the Switch version. The second would be a Nintendo newcomer. Probably a character that is appearing in a Switch game. I don't really know who I'd want. But, please. No more Fire Emblem characters. We really don't need another one. The third would be a third party newcomer. The limit's the sky for this one, honestly. If Sakurai can get Cloud Strife in Smash, then he can get just about anybody. The character I'd personally want is Sora, but Disney likely won't allow it.
2. A Melee-esque adventure mode. I know Sakurai wouldn't do another adventure mode like Subspace, so I thought: "Why not give us a Melee Adventure mode"? Melee's adventure mode was always so fun, getting to go through each Nintendo world and all. Maybe, they could even spice it up a little and there could be multiple exits leading to multiple stages. Smash 4's single player is rather lacking, in my opinion, and this would be a great way to remedy that.
3. More stages. Please give us more stages, especially ones for the lesser universes. Both bring back old ones and make new ones. Also, please allow the option to turn stage hazards off so that stages like Pyrosphere, Wily Castle, and Midgar can be tournament legal.
4. Better online. There are two major fixes that need to happen to Smash's online. One, the servers need to be much better. I can access the internet fine on every single other one of my devices. My computer, my phone, my Xbox 360, my PS4, etc. However, why is it that the Wii U gives me issues? You can't go five matches on For Glory without running into lag that makes the game next to unplayable. The servers need to be made much better and they need to do something where they pair people up based on how good their connection is. If your internet connection is good, you'll get paired up with people who also have a good internet connection. If it's bad, you'll get paired up with people that also have bad internet connections. Second, matchmaking needs to be improved considerably. I'm not trying to sound like an elitist here. I will be the first one to admit there are thousands upon thousands of people in this world who could (and would) absolutely demolish me at Smash. However, I'm more than capable of beating all the Side-B warrior Little Macs, spamming Links, etc. that are out there. So, why do I keep getting paired up with them? I know I may sound foolish due to how infamous For Glory's reputation is but, if the matchmaking actually matched you up with players around your skill level, it would be a decent way to practice.