Prince Richard from For the Frog the Bell Tolls was mentioned in the SSBM poll, but Sakurai couldn't add him, not only because he is Japan-only, but also because he wasn't as important as Sablé. They might lack certain things needed in order to have a good move set.
The Marvelous bunch from Marvelous: Another Treasure Island were mentioned in the SSBM poll, but Sakurai didn't want to add them, not only because they were Japan-only, but also probably because it would be too difficult to make a 3-in-1 character or something like that, but this was before Pokémon Trainer. Also, I don't know if they have much anything usable enough for their SSB move set.
Skapon was also mentioned in the SSBM poll, but Sakurai couldn't add him for some reason. Most likely because he lacks limbs. Sakurai also said he might not add him in the next SSB title (SSBB).
Lip was also mentioned in the SSBM poll, but Sakurai decided not to add her, not only because she is Japan-only, but also because she lacks potential. Even though, Marth and Captian Falcon lacked potential before they appeared, Sakurai added C. Falcon because he wanted to, while he added Marth for several reasons.
DeMille is a very good choice. Even though he is Japan-only and slightly popular, he has a large potential.