There's a bit of a theme to the Wii series, and I think your idea totally misses it. That theme is an abstract and intentionally unrealistic version of activities, simplified and made entertaining, with immediate pick-up-and-play appeal and the ability to improve one's performance over time through better balance, endurance, attentiveness, and/or timing.
There's no reason to do boxing again for a Wii series game (Wii Sports has a perfectly functional boxing game in it, and Wii Fit has already done a version of step-based boxing too). But a successor to Wii Fit could easily be made... There's a lot of interesting ideas that the Balance Board hasn't been used for yet, after all. Particularly in combination with other Wii peripherals.
Sky Render - Sanity is for the weak.
