As the iPad already seems to do nearly everything most users need of it, it's hard to think of obvious features to add. But there are plenty of wacky ones that would be great:
1. e-ink smart cover. There seemed to be a time when some cared about the use of the iPad as an ebook and its use in sunlight. Given the exorbitant price for covers (leather smartcover - $70!), couldn't it make sense to have an e-ink cover? If amazon can sell a kindle for $79, it seems credible that a cover can come in at a reasonable price as a lot of the work would be handled by the iPad itself. And, as a way to present infrequently updated information, it could even handle some of the widget issues.
2. video-in. iPad has a great screen that can pull in data from lots of sources. But all of those sources need to be on-board or, at best, stream from something else using a wifi network. I'd like to see iPad to iPad streaming so a single video can be played simultaneously on two screens using bluetooth or some sort of parallel ad hoc wifi. Or, even better, add the ability to allow hardware connections to use the iPad as an auxiliary screen - parents would love this in the car.
3. waterproof. As tough as the iPad is, we still take great care to keep it from getting wet. Reading in the bath, using in the kitchen, taking to the beach - all of these work much better if the enclosure is water resistant (I'm assuming water resistance covers sand as well).