We're having conversations, but it's still very early; and we're not necessarily looking at explicitly transferring the existing accessibility guidelines into a single requirement - but some aspects of them, yes. Even the existing accessibility guidelines only require that a theme has accessible default contrasts or a clearly labeled color scheme that's accessible; we don't want to restrict people's imaginations, but we want people to be able to obtain accessible themes easily with wide choice availability. And yes - where something should be fixed in core, we'd rather fix it in core.