To make Punch-Out!! a great boxing game, Nintendo needs to find the right balance between realism and cartoony pugilism. Wii Sports Boxing includes a very basic combo system -- timing your strikes properly is important, but ultimately there's not enough depth to keep this satisfying for long. I'd love to see a deep combo system in Punch-Out!! that develops as your character gains boxing experience through skill progression (super boxing RPG!). Nunchuk and Wii Remote moves could correspond to a wide variety of short jabs, side-smashing hooks, brutal crosses and wicked uppercuts.
Imagine the classic button combinations of your favorite 2D or 3D fighter transferred to the boxing ring. Unique combinations of punches could trigger specific combos to really dish out the hurt, and as a throwback to the original Punch-Out!!'s star power, super moves or more powerful punches (such as a game-winning sucker punch) could require a complex series of punches as a buildup, rewarding gamers for accuracy and skillful fist-placement over random thrashing.
Instead of the usual Nunchuk and Wii Remote diagrams, take a look at some real boxing moves, and imagine them put to use in Punch-Out!!, Wiimote in hand.

Talk Boxing presents a few of the basic boxing punches you could string together in a merciless combo to bring even the toughest opponent to his knees. For more on boxing punches, stances, and defense, check out Wikipedia.
Footloose and Fancy Free
Even though offense takes the spotlight in the boxing ring, defense has its own role to play -- and Punch-Out!! for the Wii needs to nail it. Dodging, sidestepping, and blocking all need to be intuitive and, more importantly, as responsive as possible. Wii MotionPlus will certainly help with this, as the Wiimote will have a better understanding of where it is in relation to your body. Hopefully this will enable head/torso blocking with unerring accuracy, as well as make sidesteps more effective than the annoying leaning implementation in Wii Sports Boxing.

Balance board support would be even better. Its pressure-sensing abilities would guarantee far more realistic ducking, weaving, leaning... whatever you'd do to avoid that flurry of fists, the balance board would increase the immersion factor by 230%. This is a scientifically-proven statistic.
Options and Scalability
Okay, so not everybody has a Wii Balance Board (they're not easy to find, after all) and Wii MotionPlus will cost some extra dough. That means that Punch-Out!! will have to be scalable to the level of technology available to each individual gamer. It's just not feasible to design a game that would cost $200 to play, but the options need to be there to really take advantage of the technology.
Outside of a predictable single-player championship circuit mode, I'd love to see Nintendo take the ideas they implemented in Wii Sports Boxing to the next level. I found myself working the bag far more often than working over an opponent, simply because it was a better source of exercise. Throw in a training mode, some versus action, and an exercise mode for the ultimate win -- assuming the basics are done right. And we all know they could be, if Nintendo puts the work into it.
Punch-Out!! has been gone too long, as evidenced by how excited we get when a sequel is even rumored. We got six Rockies in 30 years, but we haven't seen a new punch out since the Super Nintendo days. Give us our Rocky Balboa, Nintendo; you know we want it. Until then, hopefully Facebreaker or the bizarrely titled Shape Boxing Wii de Enjoy Diet will fill the Punch-Out!! void.
