how many of them are feasible for production and use?

... all of them? Why wouldn't they be? You may not be aware, but Pebble designed the accessory port to be universal, and provides a 3D printer file so you can make propotypes at home. Check this out, it's very cool: http://developer.getpebble.com/smartstraps/

Considering the aftermarket explosion that happened with just the original Pebble, I would assume people are already working on these.

Solar powered watch bands are out at this point.

Why? 15 minutes of charging on my current Pebble, over USB power (~500mA) gives me about 12 hours of use, since my Pebble (with my heavy usage) lasts me about 5 days.

An attachment for solar charging is not unreasonable. The panels don't have to be in the band - it could be a port that you plug the band into - just like the solar powered phone chargers.

It could also just be a micro USB port that you hook existing Solar and external battery pack chargers too.

Batteries in the band? Most likely also out.

This one is completely feasible! The 140mAh lithium battery in the Original Pebble is very tiny, you can see a picture of it here for scale.

So with just a "bump" in the watch band you could double Pebble's runtime! Furthermore you can now make flexible, lithium batteries, as well as curved. That's why products like this exist!

So it's very feasible that the entire band could be a battery. This would be awesome for hiking/camping etc where you might not be near power for weeks at a time. Such a band could have cathode lighting so it always glows or LED lights for use as a flashlight too!

Power consuming options? There goes your touted 7 day battery life.

Not at all, as mentioned, the bands can have their own batteries too. Also, sensors tend to be very energy efficient. And they don't have to be running 24/7 - a GPS band for instance only needs to be on while you're using it.

Also, the straps are designed to be used with quick release pins, so that you can swap them out whenever!