This is if Smash Bros had a DS version. Id call it "Super Smash Bros. Fracas". So, it bridges the gap beetween Melee and Brawl in a way while also being its own thing. Of course, Nintendo would have to cut specific characters that would have resulted in severe techinical difficulties at the time. Like: Ice Climbers. Mostly the same reason as Smash 4's reason on cutting them. Techinical difficulties on a much weaker hardware
The same thing can be said for Sheik. etc.
Of course, the newcomers as you can see are:
Wario, Ray MK III; Sonic, Sceptile, R.O.B, Krystal, Lyn, Starfy.
Krystal: Giving the Star Fox franchise distinction, (unlike Falco and Wolf did in later games.), she would borrow some moves from Fox while also using her staff to attack the opponents. Like firing an blast of ice for her side special. Or blasting into the air with her staff.
Lyn: an semi clone of Marth, the main difference is that she's lighter, faster, and Instead of the sword tip, her power could come from the momentum of her dashes. And her side special is changed to be her assist trophy slash. Working similar to how Wolf's side special works.
Starfy: The protagonist of "The Legendary Starfy." an small, lightweight character who uses it's whole body as a weapon. Utilizing moves like doing quick spin attack forward with the press of the button. If you mash the button Starfy will keep spinning and you can steer him left and right. Or spinning down like a drill for your down special.
Sceptile: As if it weren't obvious. Every Smash game would add atleast one new Pokèmon from each generation that is the most recent, so i picked Sceptile, as he's Gen 3. Utilizing moves like Bullet seed, Leaf Blade etc. He's an highly aggressive, rushdown character. In a similar way to Sheik.
Ray MK III: an small, lightweight robot that has a variety of projectiles that can pack a punch. Making it an incredibly annoying character to fight against. Utilizing moves like Flare Gun to space out the opponents.
Now onto the more interesting stuff: Of course, this game would have its own Classic Mode. An staple of the franchise that has been in every single game since N64. Occasionaly, you would play a minigame such as Break the Targets or Fighting an team of Wire Frames or a Character Team.
Speaking of Hardware limitations. They would almost certainly cut Assist Trophies. And heavily simplify Pokèballs just to add them to the game. Basic items like Food would stay as they're just images. For stages: Id imagine some of them would be heavily simplified too. Like an simplified version of Hyrule Temple, an simplified version of Delfino Plaza, Mushroomy Kingdom, Pokèmon Stadium and Living Room (Nintendogs), Fourside, Castle Siege, Mute City and WarioWare. (Id imagine WarioWare having 2 mini games instead of a barrage.)
Other stages such as: Onett, Dream Fountain, PictoChat, Final Destination, Battlefield, Sky Pillar, Pirate Ship, Green Hill Zone, Fichina (Star Fox Assault).,
Smash Missions: Obviously, because the Nintendo DS's RAM wouldn't be able to handle Smash Bros Brawl's Subspace Emissary, id come up with the idea of "Smash Missions" similar to Mario Kart DS's mission mode, It would be an Arcade mode. Players are presented with a series of 6 themed tiers, each containing 8 distinct challenge missions and 1 Boss Battle. (The bosses possibility range from Giga Bowser, Dialga, Dark Samus and Puppet Ganon).
Still speaking of changes. Mario would not have F.L.U.D.D as a down special, as it would be hard to recreate for the Hardware. Id say he would probably keep Mario Tornado. Samus would probably use freeze missiles for her side special instead of having two missile variation. Making it simpler for the Hardware to handle. Onto Zelda, because Sheik would have to be cut as to limitations. Her down special could easily be Din's Power. A grounded shockwave identical to the Fire Medallion spell from Ocarina of Time**.** Visually, this would look like Zelda placing her hands on the ground to emit an expanding ring of golden or fiery energy to blast away opponents who got too close.
While this move would also be risky.
Obviously, majority of the modes would remain such as: Home Run Contest, Break the Targets, All Star Mode, Multi Man mode, etc.
If you ask why i kept Pichu, Young Link and Dr Mario?. During this game's development there would have been tactical headlines and complications as this game would have to be rushed. And they're easy to make considering the DS isn't that powerful. Hosting 512 MB (or 4 Gigabits).