I'm a huge fan of Ms. Pac-Man as an Echo fighter. Someone who could play nearly identically to Pac-Man, but is still important enough on their own to be worthy enough of a player slot. Kind of the perfect candidate for the concept of what an Echo character should be.
Her one difference would reflect her own game, and that's the "fruit" she throws out 'Cherry, Strawberry, Orange, Pretzel, Apple, Pear, Banana', with the new fruits having new gameplay properties. The pretzel could fly in a pretzel shaped trajectory, the pear could act like a timed grenade, and the banana could have the properties of the new banana gun weapon, and explode on impact, which would make for a nice Key replacement as a final powerful fruit item.
Of course, they'd have to give her unique animations and such too, and maybe her taunt sprites could be different (would be EXCELLENT if they referenced Midway arcade games, but since she's a full Namco character now, I'd only expect more Namco things). And I'd hope they'd keep her look classic, no gaping mouth when she's not in her chomping form, just a little smile with her lips. Also, she's gotta have the lashes, the bow, the mole, the heels, maybe even the long gloves. Really give her that iconic Ms. Pac-Man look, because, well, she is an icon herself separate from Pac-Man, and many gamers from the arcade days would certainly see her as a worthy addition to Smash Bros.