A good start would be to introduce speed and cadence sensors to make the ipod bike compatible. Beyond that, I would like to see integration with wireless heart-rate, maybe some type of power sensor. (maybe integration with PowerTap or SRM.)
Of course, all of this would a lot easier for a company like Polar to do since they already have all the sensors available (although it might seriously cut into their head and watch unit sales).
With the SDK now available, it would not surprise me if someone builds a plug-in receiver unit to make the ipod compatible with the vast array of bike based sensors already on the market. No need to really reinvent anything, just a need to work on the compatibilities.
The benefit to working with the iPhone / iPod as a base is the massive amount of storage and user-interface superiority to any of the bike based fitness measuring equipment on the market.