Isn't M2K already going to be playtesting it or something along those lines?
I'm not too sure on that. Still, any type of voice will go a long way in getting the ideal Smash Bros game.
I've gotten a small list on what needs to be addressed the most:
1. It's safe to say that everyone wants the game to be faster in general. Faster ground speed, faster fall speed and the like. It doesn't have to be Melee 2.0, but a little more speed wouldn't hurt.
2. Planking has to go. One of the main reasons Metaknight was OP was his ability to camp to no end. Getting rid of invincibility frames when letting go of the ledge will go a long way in preventing that type of stuff. We already have a way to safely get back onto a ledge.
3. Chain grabs have notoriously prevented characters such as Donkey Kong and Bowser from going anywhere in the tier lists. They have to go in order to balance those types of characters. The Ice Climbers can be exceptions because of how much they depend on it, but even they shouldn't be able to do 0-death combos.
4. Stale-move-negation should no longer affect knockback. It's arguably have it's benefits, but it was downright detrimental for a lot of the roster (Sheik, Peach, Pit, Sonic etc). The developers could allow everyone to have the same number of KO moves instead, but then there would be no variety. Having one or two reliable KO moves should be more of a challenge for the player than a hindrance that prevents a character from being viable.
5. Less landing lag in general. The main reason so many people want L-cancelling back is because how punishable landing on the ground is. Doing so will help make aerials a lot more viable in the game. It doesn't even have to be L-cancelling if you want it to.
6. Defensive options in general need a nerf. That's one of the biggest complains from Brawl was how effective powershielding and camping is thanks to Brawl physics. I'm not sure how this should be nerfed, but it should be addressed.
Unfortunately, I don't really have a Wii U, but I hope it's a start for most people. I could also do character specific changes later on.