A) It is possible, to my knowledge, to build a sidechain on Bitshares. Really anything is possible if you can code it so.
B) We should not go onto a BTC sidechain, we should maintain our own. We have distinct features that make us better than BTC.
C) That said, they do have the network effect in terms of being the currency of the internet. With THAT said, BTS is working to be the new standard for decentralized autonomous companies. We seem to be in a competition with Ethereum on that round. MaidSAFE too.
D) There are many features that other coins can offer that we cannot. For example, extreme privacy features would severely hamper scalability in the BTS ecosystem. Ring signatures are not cheap, and zero knowledge proof relies on one node to keep the secret (though it's very very solid if that one node can be trusted).