Functional improvements a better choice
I'm not sure multiple case colors makes as much sense for iPhones as it does for the iPod touch. The latter costs more and is mission-critical, so most users put it in a case that can be any color they want. Apple would be creating inventory hassles for itself with little sales payback.
A variety of screen sizes does make sense. A larger (wider and taller) screen would be great for those who use their smart phones as a mobile office. If Apple doesn't make one, they'll lose sales. But varied screen sizes clashes with one of Apple's basic design principles: limit the screen size options so designers can take the time to make their apps look good for each of them.
Apple would be well-advised to consider having EL (Extended Life) models for all their mobile products, including laptops. Like a lot of others, I use a Mophie battery/case with my iPhone, but because it has to have two case walls, the result is thicker than it would be if a larger capacity battery were put inside a slightly thicker iPhone.
The same is true of laptops, particularly the MacBook Air. Given the bags we carry them around in, a EL model of the MBA that's a bit thicker would make no difference and having a work-all-day battery life (10+ hours) would be a major improvement and a big selling point. Past about 12 hours, a laptop's battery life becomes essentially infinite since users can recharge overnight.
Keep in mind that Apple's competitors will be looking for functional deficiencies in Apple's product line and offering models that do something users want enough to break with the Apple eco-system. In comparison to that, style issues like color are trivial. If you want a purple iPhone, you can get a purple case.