I have high hopes for SSB4, and hope it will be a very good game. I assume that it will follow in Brawl's footsteps and continue to make improvements that it made from Melee's gameplay.
For example, tripping made the game more fun. Adding in a random element that happens when you perform the simplest of actions, move, makes it more fun when hilarious things happen. SSB4 can follow this up by doing a few things. First, increase the probability of tripping randomly at least fourfold, as there are some matches where I don't even see people trip! It's ridiculous. Secondly, create some form of "air tripping." I mean, when you jump in real life, it's not always perfect, right? Some jumps should barely leave the ground, and sometimes you should trip upon landing. It makes for a more realistic and more fun physics engine. Thirdly, make it so sometimes, when shielding, a character drops their guard for no apparent reason. Granted, there is no real life comparison for this like tripping/air tripping, but assuming we could magically make protective bubbles appear around us, we would probably mess up every once in awhile.
Another major issue is that of the change in air dodging from Melee to Brawl. Melee had directional airdodging (and coincidentally, wavedashing) while Brawl had no-momentum airdodging, and you could do so repeatedly. I think that the best compromise for SSB4 would be to combine the two air dodges. Make it so it has little lag and can be done repeatedly (like Brawl) but ALSO be done directionally (like Melee). This is the perfect balance, brings back wavedashing, and nobody can complain about it. Also, Brawl increased survivability more than Melee, and everyone appeared to like this change. So, the combined air dodge will allow for even better recoveries, and surviving to even higher percents in longer, more intense, matches! Momentum cancelling was a critical part of this in Brawl, and I imagine this should be enhanced for SSB4. In order to do this, we should reduce hitstun so that people can do their momentum cancelling aerials even quicker.