So, now that we "have" sidechains (or at least, the project of how to build them), ethereum could be built as a sidechain of Bitcoin?
Afaict a clone of the ethereum protocol can be built as a sidechain of bitcoin. The official premined/IPO'd ethereum can't be pegged obviously...
In a sidechain you can't create new coins (not even with block mining reward), only transform coins from the main chain. In a 2-way pegged altcoin (like Ethereum or any other) you can.
Actually you can...
https://www.reddit.com/r/IAmA/comments/2k3u97/we_are_bitcoin_sidechain_paper_authors_adam_back/clhnd0i