But for this to work it has to be tethered to the BTC chain...and there we'll have our Bitcoin world war II
In fact, I foresee considerable opposition to implement sidechain hooks in Bitcoin Core. But sidechains can add so much value to Bitcoin that the change is likely to be implemented. If not, Bitcoin might soon become obsolete and give way to an equivalent network that implements extensibility via sidechains.
I wonder what happens when the regulators realize that sidechains can put privacy and anonymity back ;-)