1. Mr. Game & Watch
2. Little Mac
3. Lip
To be blunt, I really wish Yoshi wasn't dominating right now. Wario had to wait til Brawl to be in the game and it wouldn't have been the end of the world if Yoshi did too... would have been an obvious, inevitable choice. It's much more exciting for the remaining focus to extend to lesser celebrated picks like Marth / G&W were once upon a time.
With that in mind I'll at least push for Game & Watch. If he doesn't make it now I'm a bit unsure if he'll make it later. He's one of the most clever retro celebratory characters in the game, and even though I want to think in a vacuum and not be too tainted by thoughts of our real life Melee I believe him to be a really important part of that game. He takes on such an ominous, mysterious role in that game specifically.
Little Mac and Lip get the other nods. I nominated Little Mac and spoke on him before, we could use a more grounded fighter with a traditional boxer kit to contrast how dynamic, magical and silly some of these other characters have been. Lip is kind of the antithesis to that, but I'm ok with that too. I would have loved to celebrate, at the time, Nintendo's own signature puzzle game series and I think a fun moveset utilizing falling blocks would have been achievable even back then. I dig Pit and Sukapon too, and Takamaru albeit a bit less so, but they're just missing the boat here and I suspect Pit will be a no-brainer for Brawl. Saki and Ray are great, although maybe a bit contemporary for where I'm looking for now.
Ayumi... I unno, she's fine, but the more I think about it the more I've never really heard anyone sell me on a convincing moveset for her. At least not anything that would be strikingly distinct from what we're getting here with Peach or, potentially, Mr. Game & Watch. She'd be another silly prop kind of character, which is fun in its own right but I think there are more efficient ways to explore that / already being explored in the game. Any kind of deep mystery gimmick, looking for clues or whatnot doesn't seem achievable with Melee's general design philosophy and feels more at home in something like Smash 4.