Hey there! Looooong time reader, first time poster.
Anywho, here are my ideas for the next SSB.
1) Add a "Smash Meter." The Smash Meter fills by the player performing special moves (each special has varying amounts of Smash Points now), by performing a full taunt (though taunt spamming would not increase Smash Points), and by blocking an attack. The Smash Meter would deplete every time the player attempts an unsuccessful smash attack, is hit with a smash attack, or self destructs. The Smash Meter does not reset with a KO.
2) Add "Super Smashes." Super Smashes are gained when a Smash Meter is totally full. Each player has one Super Smash... these are stronger than special moves, but not as strong as Final Smashes. A player begins to have small red stars falling from his/her body once a Super Smash is available. Super Smashes are achieved by pressing shield + special. An example of a Super Smash would be Jigglypuff's "Deep Sleep," in which a Sleep move that fills 3x the area as a normal Sleep is activated, which also causes the affected character to not wake up for 6 seconds regardless of being hit. Players would be unable to take any fallback during this sleep.
3) Final Smashes can still be gained by breaking a Smash Ball, however, there is now a skill-based way of obtaining a Final Smash (so that it is available in competitive play). If a player keeps a Super Smash (full Smash Meter) for 25 seconds, the Super Smash becomes a Final Smash, causing the screen to go dark as per the norm. Only one player can have a Final Smash available at any time, so holding a Final Smash also prevents other players from receiving one. Final Smashes are now activated using shield + special.
4) Air Throws are now added to the game. Only some of the characters have the ability to Air Throw, which is a move that has very high priority. Some examples of air throwers are Sonic, Jigglypuff, and Luigi.
5) Added characters... I'm not going to post the ones I would like to see since that always just turns into a debate. I think 40-42 characters would be a good number. I think if you go beyond that point, it just gets too cluttered.
6) Rebalance, rebalance, rebalance. The next SSB should be made for the competitive crowd. The reason for that is that a balanced game for the competitive crowd would not negatively effect the party crowd at all (which is the larger and more important crowd from a sales prospective). Creating a game that appeals at the casual level is only aided by a game that is very well balanced for hardcore players as well. There will always be tiers, but the difference in playabiltity between a Meta Knight and a Ganondorf should not be so great.
Obviously the Super Smashes would help in rebalancing, as would the abiblity to gain Final Smashes through skill not luck.
7) Changes to some of the classic characters. One example that I can cite off the top of my head, is I would like to see Mario become a jack of all trades type of fighter. His down special would now change him into different forms (Mario, Tiny Mario, Racoon Mario, Dr. Mario)... each would have a different neutral special, a different Super Smash, and minor attribute differences (such as Tiny Mario being much lighter and running faster, but being weak). Some changes like these would make the game fresh. Obviously not every character would need such a makeover (I think Kirby's movelist is pretty good as is).
8) Bring it out soon. Make the next SSB for the Wii, and make it soon. Build it off of the same enging we've got now and just plus the heck out of the game. Balance the characters, add a few new ones, add new stages, and add a new mode... but go ahead and get the game out there for all of us to enjoy. This is not a game that needs a new system for the next iteration.
9) Online play is a must from here on out. Yeah, it was there in SSBB, but it also sucks. Go all out in an online mode that lets players play on dedicated servers. Allow players to gain awards for online wins with each character.
10) Take out the trips, they're pointless.