Nickname: AKC12 (you can go to google, type my nickname, and find some of my posts on gamefaqs, smashboards, and so on)
OK, since Fox is my favorite character, how about adding his rival Wolf Mcdonald (correct the last name please) join. Krystal would be a unique character for her magical staff (and we need more females : )).
Since you added Snake from MGS, it would be logical to add other non Nintendo characters with closer relations. Like Sonic, Knuckles, Tails or Shadow.
Ganondorf in SSBM was basically a copy of Captain Falcon. I'm sure several has said it, but give him a new moveset with the sword we see when he wins.
F-Zero GX is one of my most played games for the Gamecube. Samurai Goroh, and/or Black Shadow would be the best to choices from the F-Zero world. Maybe Deathborn as a boss.
Diddy Kong would be cool, his recover can use his rocket from Mario Power Tennis. Also Mewtwo was very weak in SSBM. Maybe his smashes should be faster and a bit more powerful with the same throw strength.
I never knew them much, but adding Megaman and co. can definetly get me more into the series.
Bowser needs help of his child (or children if you want to bring the koopa kids from super mario 3 back) so he won't lose to Mario (ok he really won't beat Mario), but it'll be interesting to see how he'll work.
You added Metaknight, so how about Kirby's biggest threat, King Dedede. (not sure on the dede spelling)
The Tales series could have a few main characters in it. Like, from Symphonia, Lloyd and Kratos, and maybe Collette would be my top choices.
Maybe there could be a Dark Samus, or you can think about adding the hunters in Metroid Prime from the DS.
Daisy can be good if her tomboy'ish style is implemented.
I would also like Raichu in it, so people know that Pikachu can actually evolve again. You already have Pichu and Pikachu, why now Raichu.
I'm thinking a different boss for each series or character. For example, Fox's would be General Scales, or Mario's would be Giga Bowser. Other bosses could be Ganon, Andross, Metroid Prime, Deathborn, Oni Link, any boss or ownage type character.
Don't forget, Stamina mode with the option to adjust the HP.
Can there be replays of short matches, or from one player mode, and save them. Or take pictures from the pause to remember some moments.
Supermoves, I want it to have NO cutscene, or at least make it optional. Also make it so it's not an automatic KO, make it hard to hit, the player can dodge it. Lots of lag to release the attack and after the attack. You can use an item to activate it, but it would be sweet if there is a condition which is very hard to meet, but if you do, then you can activate it without that item. More than 1 supermove for each character would be awesome. Only once a match for one character.
Keep all the stadium events, homerun contest, break the targets, multiman melee. Make the Classic, Adventure modes longer, and have some sort of story. Make it less linear, different endings maybe. Make race to the finish a stadium event.
Classic stages from the first 2 would be awesome. A sound test with this games music, and classic music would be awesome. Different costumes, like different colors. Mario would also have the Fire Suit, or the Super Mario Sunshine water pack. Maybe have most characters have more than 1 move set. I know what I'm asking seems way too much, but it would be the best thing in my life if it was close to it.
'Wavedashing', where you can slide on the ground by airdodging towards the ground is a very important aspect to tournament play for Melee. I hope it allows it and make it similar to execute.
I hope you consider some of my suggestions and hopes. Good luck making this game, please make it so it lasts decades, so lots of event matches, and lots of custumization would also be important. Create a character would help with this. And online play, and.... ok I'll stop so it won't take you hours to read my suggestions.
I know, it's pretty long (ok very long). But this is like 90% of what I want. So I'm basically giving all of my possible ideas which could be added.