What do you mean "garbage-collecting the chain"? I've never heard anything referred to as that. Does it just mean: Define a minimum balance, and let miners reap dust? Can your garbage-collection proposal where most of the block chain is eliminated be compatible with side chains?
I have nothing useful on side-chains yet. It is in my todo queue.
By garbage collecting the chain I mean eliminating all of the historical transaction data, and distilling the information content down to the minimum that is required to operate the coin: The association between controllers and the summed outputs which they control. Currently, almost no one is mixing, and the chain is growing about 2.3 gb/year. If it were transacting actively and mixing was in heavy use, I can see Moore's law running out of runway long before Monero ceased to be important. Definitely in my lifetime.