All Donkey Kong characters are owned by Nintendo. Diddy is a playable character in Mario Kart  Double Dash and in other Mario titles post Rare sale. (Even Dixie was a playable character in Mario Baseball). So it is very likely they will be playable in SSBB. Another exmple is Krystal from Star Fox Adventures (Rare's creation), which later appeard in Nintendo games like Star Fox command.

This are my ideas:

1- After performing the Super Smash move of some characters, they transform into something. For example Samus Super Smash is the great beam, and after that she becomes Zero Suit Samus. Link could transform into Wolf Link with Midna on top, Mario in FLUDD Mario, Luigi in Vacuum Luigi, Zelda in Sheik or Tetra, Pikachu in Raichu (lol) and if Sonic is in the game (hope so) his transformation is Super Sonic.

2- At least this game must have two characters per franchise, (except those cases that are impossible like Ice Climbers) because it can't be possible that when SSBM was released here there were two uknown characters from an unknown franchise in America (Fire Emblem), and Donkey Kong which was well known had only one character (Now they must add Diddy Kong and King K. Rool).

3- More Nintendo characters than ever with new franchises: Pikmin, Animal Crossing, Ballon Fight, Mach Rider, Project HAMMER, etc.

4- If there are more third party characters my personal favorites are: Sonic (sega) and Megaman (Capcom). From Sonic the following characters: Sonic, Tails, Knuckles and Dr. Robotnik.

What do you think?. 

My characters suggestions are: Diddy Kong, King K. Rool, Dixie Kong, Kiddy Kong, Mach Rider, Ridley, Punch Out! characters, Olimar, Louie,the guy from Project H.a.m.m.e.r., Kotake & Koume (Twinrova), Darunia, Princess Ruto, Battalion Wars characters, Ash, Misty, Brock(lol), Celebi, Latios & Latias, Mew, Andross!( Somehow), Krystal, Wolf O' Donnel.

I don't want any character to go, and at least two characters per franchise so they can battle with someone from their own game.