It seems as if the general buildup of the Device is like it was with the iPhone 4(s)
there are 2 screws on the lower end which lock the backplate in place and when removed release it for easy access and maybe replacement of the back?
so replacing it with something more sturdy seems legit? even though production might be a bit complex it definitely seems possible as it did happen for the iPhone 4
A cool idea is the solar back panel, but it would surely interfere with Qi and NFC? so not a realistic possibility, but a PC/Plexi/Kevlar back might very well e on its way from 3rd party suppliers.
If google sells enough of these little beasts demand for replacements will be striking? even if it is sturdy? which it does not seem to be as we have already seen some cracked backs
also some black rough anodized aluminium might be a good idea (not the glossy/flat stuff Apple uses but traditional as it was on the HTC Legend and is on the OneX DesireS??)
Carbon Fiber is the best solution. That or a nice billet aluminum. Carbon fiber is extremely strong, extremely light weight(depending on weave/construction), it is a bit more expensive than plastic, but it isn't ridiculous. Plus it looks VERY nice and many options in looks with CF.
CF would make this product PERFECT looking I think.