On the idea of a sequel to the SSE, here' my idea for a sequel (all playable characters should be included; I'm looking at you Jiggs, Wolf and T. Link):
WARNING! THIS IS A VEEEEEEEEEEEEEEEERY long post.
A backstory is set up telling the tale of how "This World" came to be.
In the beginning, the Master Hand and Crazy Hand really had nowhere to dwell. The universe was nothing but a swirling mass of Shadow Bugs, which had an amazing attribute. The ability to create life. The Hands are God-Like figures, with unknown origins. The Master Hand began creating worlds like the Mushroom Kindom, Hyrule, Dream Land, Kanto, and made inhabitants for those worlds to keep him company. His first creation was R.O.B. (Ancient Minister). The Crazy Hand also wanted to have subjects to call his own, so he attempted to make his own creation. He was unskilled in the creation of life, and was only to make a 2-D, featrurless, bland figure, Mr. Game & Watch. He disowned Mr. Game & Watch, ashamed of his own inability to rival the creations of his "brother". The Master Hand, feeling sorry for Game & Watch, took him in and gave him a world to live in. The Crazy Hand gave it one last shot, and made the creature we now know as Tabuu. Tabuu looked alright, but he still didn't meet the Crazy Hand's ridiculous standards. The Crazy Hand, so frustrated and jealous as to the success and popularity of his brother, went mad, and dedicated his life to destroying the creations that helped his brother overshadow him. Tabuu, hurt by the disownment of his creator, took on a similar task. He, however, vowed to destroy the Master and Crazy Hands, and rule over the lands created by the Master Hand. He was soon discovered though, and the Master Hand, Mr. Game & Watch, and R.O.B. joined forces to banish Tabuu to a world where he could cause no more trouble, known as Subspace. The Crazy Hand continued his effort to antagonize the Master Hand. A The two Hands fought for years, and finally decided to stop the fighting for thier own sake. The agreement was that the Master Hand would create a place for the Crazy Hand could run rampant, and destroy as many things as he wanted, and he would also create a place for peace, and unity. So he created a new, gigantic world as the solution for both halves of the promise. It was split down the middle by a line. One half would be for the Crazy Hand (to be called "That World"), and the other half would be for R.O.B. and the Master Hand (to be called "This World"). For a while this agreement worked, and all was well. the Master Hand started what we know as Smash, and sent out invitations to inhabitants of worlds he had created. These invitations challenged recipiants to come to a world (This World) that offered everything. They could vacation here, meet friends, or just go sightseeing. But the real reason for coming to This World was to test yourself against your creator, the Master Hand. This event was not intended for evil; the Master Hand didn't want to harm his creations. He just wanted to check up on them every once in a while. This cycle went on for quite a while, until Tabuu struck. Once the Crazy Hand heard that Tabuu was wreaking havoc on This World, he fled. Nobody knows where. He was afraid that Tabuu would eventually reach That World. But as we know, Tabuu never got that far. After the Subspace incident, the fighters in Brawl stayed to help R.O.B. and the Master Hand to rebuild the Island of the Anicients, as well as help heal the wounds that the Master Hand sustained from the Chains of Light. This task was completed, and the fighters returned to their individual worlds. Word reached the Crazy Hand that Tabuu was dead, and he also returned to This World.
Main Plot for SSE Sequel
It's back to business as usual for This World. The Master Hand has been working to create more R.O.B.s, since many were destroyed during the Subspace incident. The Master Hand has sent out a new batch of invitations to a host of veteran and new fighters. Since recent events, the Crazy Hand has started thinking about his situation. He notices how lonely he is, and how empty he feels. He devises a plan to destroy This World, and hold several other worlds hostage on the demand of absolute power over the entire universe.
He sends out his own invitations, but to the villians. Bowser, Ganondorf, Wario, Wolf, Dedede (and according to my self-made roster,) Waluigi, Team Rocket, Mewtwo, Black Shadow, K. Rool, the Black Knight, Ridley, Weavel, Vatii, and Bowser Jr. all recieve an invitation inviting them to "Be a part of the greatest undertaking ever". Dedede and Wario do not accept the invitation, trying to aviod another Subspace travesty. Mewtwo accepts, and through an agreement with the Master Hand, acts as a double-agent. All of the other villians do accept. Some NPCs that the Crazy Hand invites, or otherwise abducts are Eggman, Porky, Petey Piranha, Darkrai, Raquayza, Zant, Andross, Medusa (Kid Icarus), Kamek, Empress Bulblax, Polar Bear King (Ice Climber), Omega (Megaman), Metal Gear RAY, and the INK'T Colonel.
The Crazy Hand also managed to get ahold of the remains of Duon and Galleom, and synthesized them into a hideous creature, called Subspace Mk. II.
Several of the villans have second thoughts during the planning process of the attack.
Team Rocket (in typical style) flees to This World and joins the heroes after discovering that the Crazy Hand plans not to turn the heros into trophies, but kill them.
Bowser Jr. also feels uneasy about this idea, and tries to convince his father to abandon the plan. After failing to convert Bowser, Bowser Jr. also flees with Team Rocket.
Waluigi is the last of the villians to have second-thoughts, and ends up escaping with Bowser Jr. and Team Rocket.
All three of the above villians escape with Mewtwo's help. However, Mewtwo is seen helping these villians, and is turned into a trophy.
The villians begin to develop several technologies to defeat the heroes. They decide to test them on the heroes home planets. This brings the heroes back to various familiar worlds (Hyrule, Mushroom Kingdom, etc.). Every world has a main playable villian to defeat, as well as an NPC Boss, who once defeated, return to That World.
Once they have been driven out of every world, the villians are flustered, and decide to develop technology (out of old Subspace weapons) to corrupt heroes. They test it out on Mewtwo, and it works, but not how the villians expected. Mewtwo becomes truly corrupted, hating good and evil. Mewtwo destroys the villians' base in That World, and heads off to destroy This World.
The villians, left with no base and no more options, decide to follow closely behind Mewtwo and attack after he catches the heroes off guard.
Meanwhile is This World, since all of the heroes have returned from battling the villians in the other worlds, everyone is preparing for the villians' attack.
The final battle takes place at the border between This World and That World. As the heroes are braced for battle, they see Mewtwo coming over the horizon. Not knowing of his corruption, they let thier guard down.
However, when they see the villians following closely behind, they realize what's happening.
At this point, the player can choose a side: Good or Evil.
Since there are more heroes than villians, if the evil side is chosen, a handicap will be in order.
Like the Great Maze, you pick 4 characters to face all of the characters on the opposite side. Also like the Great Maze, you will be able to switch out characters every so often, and recovery items will be available on occasion.
Regardless of which side you choose, the following cutscene will play after the final battle:
Both sides stand opposite of each other, looking exhausted. All of a sudden, a great explosion occurs high in the sky, and everyone looks to see what it is.
What remains of the explosion is a giant purple sphere, one that resulted in the use of a Subspace device.
Several heroes and villians gasp, thinking that Tabuu was dead, and Subspace was in the past.
From the Subspace sphere floats a severly mutated Mewtwo. A hole has been blown through his chest, and in the hole floats a small black sphere (the one Tabuu had in his chest).
A transparent Tabuu appears behind Mewtwo, arms crossed. Flashbacks show that after the Subspace Emmisary, Tabuu's "heart" (the black sphere) was blown to That World by the explosion of Tabuu's death. This "heart" was unknowingly used in the machine that the villians used on Mewtwo after it was found by Ridley.
MuuTuu is what this hybrid of Mewtwo and Tabuu is called.
To demonstrate his power, MuuTuu shoots "Off-Rays" (not quite Off Waves because they don't cover the whole field) at each side of the confrontation, turning some characters into trophies.
R.O.B. dons his Ancient Minister costume, and the Master Hand and Crazy Hand act as his hands.
Good and Evil join forces (because everybody hates Tabuu).
Here's where it gets special.
For the Final Final Battle you control the Ancient Minister/M. Hand/C. Hand character. During this fight, you control this specialized character (I'll think of moves later) and both you and MuuTuu have a health bar. All of the other heroes and villians occasionally come on-screen, attempting to hurt MuuTuu, but their attacks cause very little damage, and they are easily dispatched.
After you defeat MuuTuu, the following cutscene plays.
MuuTuu slides across the ground, presumably from the last hit you delivered, and looks up, appearing ready to fight again. MuuTuu clenches his fists and grows twice his size. It looks as if you will have to fight him again.
Suddenly though, a Smash Ball materializes. Everyone stops and stares in awe at it as it daintily floats toward the Ancient Minister.
The Master and Crazy hands meet in an enourmous clap on the Smash Ball, breaking it.
The trio glows, and stares at MuuTuu for a moment.
Then the Master and Crazy Hands perform a famous move: The Master Hand flanks MuuTuu on the right, and the Crazy Hand on the left. The two hands pound fists, catching MuuTuu in the middle, and the screen goes white.
Afterwards, a scene fades in with all of the heroes and villians standing together.
The Master and Crazy Hands shake hands, and the Crazy Hand flies away into the sunset. The Master Hand flies up to the Island of the Ancients, and all of the characters are left standing, watching the hands fly off in opposite directions.
Mario, Link, Pikachu, and Kriby step forward in front of the crowd.
After the hands are out of sight, Mario says, "Let's-a Go!" and the screen stops on the entire cast running after the four.
*NOTE*
The real meat of the gameplay is the exploration of Nintendo Worlds by the heroes.
To make this portion of the game long enough, there'd have to be several worlds.
Distant Planet, Mushroom Kingdom, Hyrule, Dream Land, Kanto, just acutal places from games.
I call it... I can't decide. You think of a name.
I like "War of the Worlds", but that's kinda already taken.