The Blockstream team has made it CLEAR they have NO interest into alternative currencies built on a sidechain. This is absolutely not their vision of sidechain and it is explained repeatedly in the white paper.
Adam Back
sidechains are a generic extension mechanism. we hope many people make use of the sidechain extension mechanism to add innovative new features centered around the bitcoin currency.
from the White Paper Appendix C
Atomic swaps
Once a sidechain is operational, it is possible for users to exchange coins atomically between chains,
without using the peg.
It sounds like altcoins to me.