Brawl had a few innovations that were noteworthy, but unfortunately the entire game was completely mangled by a lot of crap, like random factors(tripping, input delay, others?), no hitstun to speak of, horrible ledge mechanics(EVERYTHING), a general slow paced feel, and Metaknight, depending on how you look at the game.
Melee definitely did outshine Brawl in a lot of regards as far as gameplay is concerned, but there were a few quirks about Melee's controls that made it feel a lot... shall we say "stickier" than Brawl did later on. Characters seemed stiff to the touch, shield mechanics were bit wonky and unintuitive, etc. I understand that the competitive Melee scene was able to power through all of this and master the controls like they were nothing, but we don't necessarily need Smash 4 to be as... "difficult" to control as Melee was. Not to say that difficulty is a bad thing, of course, and I always welcome a good challenge, but I see a distinct difference between challenge and having to break your hand on your controller pressing 15,000 different buttons over the course of 2 frames.
To that end, I feel like that the best way to model the mechanics for a Smash game would be to find a medium between Melee and Brawl, taking mainly(but not completely) from Melee in terms of gameplay, but taking mainly(again, not completely) from Brawl in terms of control and handling. Increasing the gamespeed to something between Melee and Brawl, but closer to Melee, implementing Melee's shieldstun with Brawl's shield mechanics, reverting the hitstun back to Melee's levels, keeping(but perhaps reducing slightly) Brawl's buffer system, returning Melee's 'disabling of edgesnapping while ascending' mechanic, implementing a hybrid of Melee and Brawl's airdodges(Smash + Shield = Melee airdodge??) are just a few things I'd recommend in bringing the two game's engines together. Of course, all of these choices are subjectively chosen by me, but you get the idea.
A few other changes I would recommend but are not related to Melee and Brawl would include:
- diminishing returns to ledge invincibility for each successive edgegrab performed w/o getting hit or touching solid ground
- overall reduction to ending invincibility frames in ALL dodge moves
Melee and Brawl both had their strong points(although it's pretty clear cut Melee had more), so why not design a Smash game that brings the best of both worlds together!?