I could also imagine sidechains that were explicitly not worldwide in scope, such as those limited to a particular nation or economic block.  I.E., there might be a Eurozone specific sidechain, a United States specific sidechain (but would that be redundant?) and a Francophone sidechain.  There might be a sidechain for Portuguese speaking nations around the world, or a sidechain just for nations in South America that don't speak Portuguese.

There could be a sidechain that exists entirely on Tor, using high anonymity rules; or a sidechain sponsored by governments for the expressed purpose of paying taxes (but who would join this voluntarily?)

Many people have complained that Bitcoin isn't anonymous, because the entire transaction history is visible.  Sidechains would fix that immediately, even without improved anonymity rules.