SSB64--->Melee was a big leap. We got two new grabs, a new special, air dodging, light shielding, wall jumping, spot dodging, adventure mode, event match, trophies, and other minor knick knakcs.
Melee--->Brawl on the other hand was a smaller leap. We didn't really get any game play improvements aside from crawling. However, what we did get was an online mode and a full fledged story mode. Both of those two however were unsatisfactory.
Brawl--->SSB4 would look like this to me:
1) Revamped Adventure Mode: The Subspace Emissary was flawed in the sense that it really didn't really capture the spirit of Smash Bros. It didn't feel like an amalgamation of all the represented series' worlds. Contrarily, it felt rather generic.
Personally, I believe I have a better idea for a story mode. It's called Dimensional Disorder. It goes something like this. The game is divided into three "perspectives" representing different universes. There is the space universe comprising of characters such as Captain Falcon, Samus, Snake, Olimar, the Star Fox reps, Pokemon characters, and any other futuristic like characters. There is the whimsical universe comprised of characters like those from the Mario series, Kirby Series, Sonic, the Ice Climbers, and any other cartoon-esque design. Finally, there is the classical universe, comprised of characters like those from the Zelda series, from Fire Emblem, Pit, and any other medieval-esque design.
The issue in Dimensional Disorder, is that characters from certain universes are suddenly warping into others via mysterious portals. The villains from each of the represented universes find a way to manipulate these mysterious portals in order to deceive their primary foes. So, while the heroes are attempting to decipher the cause of the portals, they are each "befriended" by a villain from another universe. For example, Bowser befriends Link, Ganondorf allies himself with Mario, and Metaknight and Dedede team up with Captain Falcon and Samus.
The story branches on from there. But aside from an interesting and involving plot, Dimensional Disorder also includes fascinating game play scenarios. While in the SSE you breezed through generic levels, in Dimensional Disorder, each level you play through is inspired by the universe it is set in.
For example, at one point in the story Samus, Captain Falcon and Fox crash land on a mysterious planet. The speed at which they crashed at, however, shrunk them to approximately the size of a quarter. See where this is heading? They befriend Olimar, a frequent visitor to the planet, who with the aide of his Duracell Battery enhanced ship, can re-size them back to their normal stature.
The dilemma for Olimar however is that a Bulborb plundered his battery prior to him ascending into the atmosphere for the night. After a cut scene plays explaining this, you are tossed into a level with Olimar and are tasked to transport the Bulborb-protected battery back to Olimar's ship before nightfall.
All levels are derivative from the represented series in SSB4. There are Zelda dungeons with puzzles, on foot F-Zero races, stealth MGS levels, and even Game and Watch inspired objectives.
2) Dynamic Online: Don't make the buffer mandatory. Allow us to go through lobbies. Allow us to change the rule settings. I don't have many issues with this though.
3) User Editors Galore: Don't just limit us to creating stages. Heck, even the stage editor was shoddy. There should be an event editor with triggers, a target test editor, a board the platforms editor, and an Adventure mode editor. Nothing adds more to replayability than user generated content, all shared through Wi-Fi.
4) Individual Shields: Yoshi already has it, bring it across the whole cast. This would drastically alter the way people approach specific characters. The shield grabbing meta game could totally be altered!
5) Hold Items: Kind of like in Pokemon. Prior to a match, you could have your character hold an item for use. You'd be limited to a certain set of specific items though. With the tap of a taunt button in a match, you could bring out a one time use weapon.
6) Aerial Smashes: Chargable attacks that work like smashes, but are only usable in the air.