Wii Sports will be the most widely owned console game ever by the time it's semi-sequel is released. Mario & Sonic has been the next closest thing with sales currently at 6M. I don't believe Nintendo has really taken advantage of the potential branding power of "Wii Sports" and has left too big of an opportunity for third parties to exploit with shelf space fodder. Here's two paths I believe they should go.
Wii SPORTS/PLAY HYBRID: A casual, recreational focus that seems to be the intention of Wii Sports Resort. The direction Wii Sports goes here is a collection of pick-up to play party games with various innovative controls but limited depth. Nintendo should follow up with a "mountain" themed Wii Sports Retreat later.
Wii SPORTS/FIT HYBRID: With the Balance Board & WiiMotion+ makes possible, perhaps, for the first instructional sports video games using realistic physics to recreate the trio of "swinging" sports (Tennis, Golf, & Baseball) well enough for real-world self improvement. It would require expert sports trainers to help program & some compromises would have to be made for playability, such as in Tennis you probably wouldn't be able to control player movement, but the potential market is a VAST. I'd get some ex-jocks like Cal Ripken to be a Mii-ized coach & therefor would be called the "Pro" Series, for example Wii Sports Pro Baseball.
