IPv6 and Bitcoin, privacy risk?
The only difference between IPv4 and IPv6 from a privacy perspective is that IPv6 will allow an external observer to distinguish between two Bitcoin nodes operating on the same network. However, if they're different versions of Bitcoin, that's already possible.

There's a second angle, which is not really Bitcoin-specific. Most SOHO routers will not allow inbound IPv4 unless a device has specifically signaled using UPnP that the router should forward a port. However, they're typically set up to allow all inbound IPv6 traffic, which would allow your Bitcoin peers to port scan you in a way that wouldn't be possible with IPv4. You can test what they would discover using this tool, or nmap.