@Maxwell: I was hoping to see a modified version of mini- blockchain scheme amongst your sidechains candidates or to say elements. Did you consider it at all?
And if you didn't, why? Is it because the mini-blockchain is not a secure ledger cryptographically? Or because it is not feasible to attach such a mini-sidechain to bitcoin?
I don't think it's all that interesting: It requires that you trust the miners implicitly for the history, and if you're willing to do that you can use SPV which is much more efficient. And, I say this as the person who originally proposed state commitments in 2011:
https://bitcointalk.org/index.php?topic=21995.0If, for whatever reason, one really is interested in an in-between mode: a side effect of elements alpha witness segregation is that you can sync the chain while skipping 2/3 (to 95% with CT) of the data, but still have perfect security for the utxo set.
The txout commitment schemes have a non-trivial cost for full nodes as the txout set becomes large, e.g. requiring on the order of 20 times the amount of I/O-- so a one time miner trusting init takes less bandwidth but then you have much higher IO and CPU ongoing; so it's not actually clear to me that they're a win; even ignoring the security trade-off... which is part of the reason that they haven't moved forward in the Bitcoin space.
Some of the other things in the miniblockchain stuff are just incorrect. Like it claims to not have transaction malleability; but it does due to the inherent DSA malleability. I don't recall other features you might have been looking for.