I partly agree with your list, but;
Mewtwo isn't going anywhere. Mewtwo is a legendary Pokemon, that's very popular and well-known, that got it's own movie.
Get rid of Roy and Jigglypuff. Jigglypuff was never imporant in the games, and isn't important anymore in the anime, and Roy was just an ad for FE6.
Get rid of Gardevoir. Come on. Are you being serious? Gardevoir is knowhere near popular compared to Deoxys, Blaziken and Plusle & Minun.
Add Lucario, Deoxys, Plusle & Minun, Munchlax, Dragonite and Charizard.
The problem with Mewtwo is that, popular as he may be, he wasn't well received by SSB players. He's awkward to play as unless you can invent your way around that, so he's got a lot working against him. And you'd be surprised how popular Gardevoir is, especially for people who don't like the Legendaries as much. Plus, Gardevoir made it onto the Toukouken list and Deoxys didn't--to my recollection--meaning Gardevoir is a more likely candidate than Deoxys.
Plusle and Minun? Are you kidding me, after all the Pichu-related griping that's been going on here and other places? Wow, it's like Pichu only there's two of them and they're more useless. Ice Climbers Pichu. I don't see it working out for some reason.
Roy and Jigglypuff, now
there's a set of characters not going anywhere. They're too widely used. If popularity and current importance outside SSB really made a difference, we wouldn't have seen Marth or Roy, Falco, Mr. Game & Watch or a bunch of other characters on the Melee roster.
In short, I stand by my list.
Edit: I decided to extend my total list by adding some stages that would be neat to see.
Super Mario Bros.:Mario - Delfino Island: Rico Harbor
Luigi - Mushroom Kingdom: Luigi's Mansion (Rooftop)
Peach - Classic Mushroom Kingdom: Super Mario Bros. Underworld
Bowser - Koopa Kingdom: Bowser's Castle
Toad - Classic Mushroom Kingdom: Super Mario Bros. 3
Geno - Mushroom Kingdom: Forest Maze
Legend of Zelda:Link - Dark Realm: ??? (TP)
Zelda - Classic Hyrule: Overworld (LoZ)
Ganondorf - Hyrule: Ganon's Castle (OoT)
Young Link - Great Ocean: Forsaken Fortress (WW)
Vaati - Hyrule: Death Mountain (FSA)
Star Fox:Fox McCloud - Lylat System: Corneria
Falco Lombardi - Lylat System: Titania Desert
Krystal - Lylat System: Zoness Sea Base
Wolf O'Donnell - Lylat System: Sargasso Hideout
Metroid:Samus Aran/Zero Suit Samus - Tallon IV: Phendrana Drifts
Ridley - Aether: Hydrodynamo Station
Kirby:Kirby - Dreamland: Mad Machine (K:CC)
Meta Knight - Dreamland: Halberd
King Dedede - Dreamland: Dedede's Castle
Pokémon:Pikachu - Kanto: Lavender Town
Jigglypuff - Johto: Goldenrod City
Blaziken - Hoenn: Pacifidlog Town
Gardevoir - Shinou: ???
Donkey Kong:Donkey Kong - Kongo Jungle: Jungle Japes
Diddy Kong - Kongo Jungle: some waterfally cavey area
King K. Rool - I dunno. I don't play DK games much either.
F-Zero:Captain Falcon - F-Zero Grand Prix: Mute City
Samurai Goroh - F-Zero Grand Prix: Silence
Yoshi:Yoshi - Yoshi's Island: Donut Plains
Kamek - Yoshi's Island: Dark Island
Wario:Wario - WarioLand: Kitchen Island
Captain Syrup - WarioLand: Black Sugar Pirates' Ship
Mona - WarioLand: WarioWare Factory
Fire Emblem:Like I said, I don't really play this series.
Earthbound:Same here.
Other:Ice Climbers - "Battlefield"
Mr. Game & Watch - "Final Destination"
Pit - Angel Land: ???
Solid Snake - ??? It looks like he's got a level of his own anyway...
Rockman/Megaman - Mega City: Cutman's Stage
Sonic the Hedgehog - Green Hill Zone: Act 1
Retro:SSB - Hyrule: Hyrule Castle
SSB - Kanto: Saffron City
SSB - Planet Zebes: Brinstar
SSBM - Game & Watch: Flat Zone
SSBM - F-Zero Grand Prix: Big Blue
SSBM - Kongo Jungle: Kongo Jungle
It's worth noting that, in my mind, all the Star Fox levels are based on land instead of on the friggin' Great Fox, which wasn't fun in SSB and remained not fun in SSBM. Same general idea for the Pokémon stages--they take place in actual places from the Pokémon games, rather than making some crap up.