I think the gamecube will have:
Extra Characters for Arcade mode.
Extra Characters for Puzzle mode.
Extra Arenas for Arcade mode.
Extra Arenas for Puzzle mode.
Extra Death traps for Puzzle mode and maybe Arcade.
Characters:
Sonya
Johnny Cage
Quan Chi
"Monster" (outfit for Scorpion hopefully )
"Ghost" (outfit for Liu Kang)
Shang Tsung
Mavado

I'd say they'll probably add the Deadly Alliance characters to the roster. Just add them in, I doubt they'd change them. And if thats not what they were planning to do well they better be reading this because this is what they should do. They said they would make up for the online play for the gamecube. In my eyes Deadly Alliance characters would make up for it.

I don't know who will be included but I will be waiting untill it gets released for Gamecube. I hope Sonya, Kung Lao, and Cage are going to be in this one!

i think all the other puzzle kombat characters (like Havik, Kira, Ashrah, Hotaru and so forth). maybe a new arena or two. but i doubt there will be more characters in the arcade. or maybe a new bonus feature, like you can wear your alt. costume in chess kombat.

Adding the MKDA characters to the roster would be pretty cool, but I think thats the easy route to "please" the GC owners. Look at MK Tournament Edition, they had three new characters in it, so hopefully GC MKD will get new characters too.

i think the gamecube version will be the same as the other except for online pay......midway is too busy working on Shaolin MOnks and MK7...y wud they spend more time on the same game....to make it better? maybe.....but i dont think they wud

I think that extra characters in arcade mode is almost a given. I dont see how they can make up for lack of online play other than putting in extra characters. Almost no one would buy it. I would think that the Ghost Liu Kang would be another costume as well. Who knows maybe they'll make the characters you can battle off with in konquest mode (i.e. shang tsung, sonya, kung lao) playable. Their model is already in the game, all they would have to do is give them moves, storyline, bio and maybe a mugshot pic for the character select screen.
Anyways I hope that Monster will be a new character. He is an interesting character.