-Sakurai wants to focus more on characters this time, and less on the extra fluff, though he still wants there to be plenty of extra content.
-Some of the assist trophy characters will become full characters, however Lucario will not be returning. Mewtwo is also out of the question, as are the Melee clones. It's also a possibility that some characters who were starters in Brawl will become unlockable in 4, much like Falcon and Ness from Melee to Brawl.
-Main control scheme will be Classic Controller, as the Wii U is not compatible with Gamecube tech. The tablet controller won't be for controlling characters, but there will be a use for it.
-Pokemon Trainer may get new pokemon. Another Pokemon Trainer character is "Highly unlikely"
-No new characters for "heavily populated" series, more focus will be put on series that need more characters. Star Fox series is "A notable exception" to the rule for some reason...
-No more Pikmin characters. Feels that Olimar is "suitable representation"
-No more F-Zero characters, as Falcon had to have a'lot of work done to become a character, and is the "face" of F-Zero. Having other characters would detract from this...
-No more Mother characters, as the series is still mainly a Japan exclusive. Lucas and Ness represent the major titles effectively.
-No more Donkey Kong characters, though Donkey Kong assist trophies are in high demand, and at least three new assist trophies are planned, including one from Diddy Kong Racing, which will be represented better in Smash 4.
-No more characters for the Yoshi and Wario series.
-No more Zelda characters; if Tingle were to become a character(NOT confirmed), he would get his own series icon and be the sole representative.
-No more Kirby characters.
-Kid Icarus will be receiving a major overhaul. Expect at least one new character, several assist trophies, and a new stage alongside the one from Brawl.
-For Metroid, there will be one new character added. One that fans requested, and was thought to be an impossible addition, but the team have found a way to add them to the game, though it won't be a perfect transition.
-Alternate costumes for characters other than Wario are "being looked into", still nothing concrete.
-No more Fire Emblem characters. Marth represents the old, while Ike represents the new. Fire Emblem is still going to recieve extra attention to other details however, including at least two new assist trophies.
-If a Third Party character has appeared in Smash, they are not likely to leave. In the case of Sonic, his reception was met positively, so SEGA has decided to give the development team free liberty to add him as a playable character. There won't be more than one character per company however. For example, if Capcom allowed the team to use both Ryu and Mega Man, they could only choose one.
-In addition to new stages, there will be a selection of classic stages from all 3 previous Smash titles. One in particular from the original was "missed dearly by the fans".
-Mario will incorporate a new move from the Galaxy series, and the Galaxy series will be represented proper in Smash 4. Galaxy wasen't represented in Brawl due to not having substantial content to work with, but now there is "plenty of content to derive from the Galaxy series, especially with two distinct titles".
Note that is is all tentaive and is subject to change. The game is not in actual development, these are merely ideas discussed by a board of nintendo employees and member of the development team. Kid Icarus is near completion and development will begin a few months after the release.