Re: ISP level caches are surely one of the first things
Not impossible to cache encrypted traffic, you just need to know the keys (which a netflix controlled cache would do)
There is no copyright trouble for a cache that doesn't make available stuff that wasn't available anyway on the network (at least not in the US, DMCA safe harbour regs took up a not insignificant amount of my employment a few years back)
I put quite alot of effort into designing such things for a previous employer - because ISP level caching is just so damned stupidly obvious - at least it is if you've worked with ISPs in the past (as had my previous employer).
Particularly for things like netflix, which are likely to have short time high value items (just released shows etc)