Sharia law for civil disputes if all parties involved agree to it. I still don't like it but as has undoubtedly been pointed out to you already, it's not unlike binding arbitration whether religious or secular.
The problem with "voluntary" sharia law in Canada, is that Muslim women don't get a choice. They "agree" to sharia, or they get beaten, or stabbed to death in "honor killings" if they insist on going outside of their family for help.