You know, people are worried that this might be a remake of the NES game rather than a full blown sequel, because the teaser trailer showed Glass Joe, King Hippo and Von Kaiser, and the ESRB description mentions possible returning characters.
Well, I think the game could include both classic characters and new ones.
Look at Street Fighter IV. Some screens make it look like a remake of Street Fighter II, especially since it features the original cast as well as "remake" of classic stages. But it includes new characters as well as some characters from the "Alpha" series.
I can't say for sure if this will happen, but if it doesn't I think I know why.
Going back to Street Fighter, one of the reasons many fans didn't feel at home with "Street Fighter III" is because only a few of the old characters returned and the cast was completely new. While some were creative they didn't feel as nostalgic as the classic characters.
You could say that Super Punch Out suffered from the same problem. While some fighters returned the whole cast was new, right down to the controllable fighter. Like in SF III, these characters were creative (some were actually ripped from the old arcade game) they didn't feel as fun as the classic cast.
I think that's what Nintendo and Next Level Games wants to accomplish with Punch-Out Wii. They know that people have more memories of the classic NES game than the arcade and Super NES game combined, so in a way it makes sense that if the franchise is going to see a revival the characters should come along for the ride.
In SF IV, the characters might be familiar, but they look, act and feel different from the older games. The new technology has enhanced their traits, making them brand new but still being the same.
Punch-Out Wii might look to do the same. The classic characters have different reactions and expressions (french bagels drop out of Glass Joe when he is defeated) but still retain certain traits (King Hippo's pants still fall down, revealing a weak spot).
So the game could be a mix of the old (characters, themes, concepts) and the new (graphics, controls, gameplay). And I might be crazy to say that this is what will make the game a winner, even if it isn't the follow up we've been waiting for 10 plus years.