They should have a make your own character option. I'd love to design a signature character.
As much as I would LOVE for this to happen, I highly doubt that Sakurai would implement that idea into the game. Being generally a "Nintendo All-Stars" battleground with guest appearances, I don't see a Create-A-Character Mode being designed for the likes of the game.
If I'm wrong and they do make one, I'd love to see the customization of different body types, and not just human like it usually is. I'm thinking the following:
-"Toon" (Toon Link proportions)
-"Orb" (Kirby proportions)
-"Human" (Mario and Earthbound series human proportions)
-"Real is Tall" (Snake and other less cartoonified human proportions)
-"Beast" (Variations such as "Reptile" and "Ape" (Bowser and DK respectively), as well as "Quadruped" and "Biped" (Ivysaur and anyone else respectively). There'd be more options as well for characters like Pikachu or Squirtle.)
-"Wackjob" (Characters like G&W: An oddball design basically)
-"Robot/Armor" (Self-explanatory)
-"Furry" (Bipedal humanoid animals, like Fox and the gang (Also can be like Ganondorf's "Ganon" mode from OoT))
I'm probably forgetting a few, but these are the first ones that came to mind. Along with those, depending on the body type you choose you could give them a weapon or two to hold at all times, with a couple in your back pocket for specials. These weapons could be swords, staffs, blasters, and even more obscure weapons such as scythes and maces.
But, as I've said, I don't see a Crate-A-Character Mode for Smash anytime soon, if ever. The most we'll probably get is a slight customization of existing characters.