Perhaps give browsers the ability to load cached content across domains? So, Google would tell my browser to cache this resource, and then when another website requests it, it's already available in my cache.

I'm sure there are some security/privacy implications that would need to be worked through, but they don't seem insurmountable? It can't be worse than letting websites show fake URLs...

(Security-wise, the primary thing that comes to mind is you'd want to store and check a hash of any asset cached by a third party, to make sure they're actually the same file.)