Any side chain must therefore be able to generate currency out of the blue and on-demand. This might be possible to do in other protocols but not bitcoin. So the symmetry is basically lost. am I missing something?
The sidechain-coin could have many different features, for instance it could be a clone of Ethereum and its "currency": "ether", being able to replicate its functionalities like issuing subcurrencies or assets (out of the blue) completely decoupled from the bitcoin blockchain. But the higher level currency (the Y issued ether) is pegged to X amount of bitcoin.
The concept is not new and is similar to the proposed spin-offs... https://bitcointalk.org/index.php?topic=563972.0