If the aim is to reduce complexity by reducing the number of bridges, then I guess you are suggesting to implement security group rules as flows on br-int? I'm not sure if this would really reduce overall complexity. I think it just make the code that manages br-int and the flows on it very complex instead.

Is the hyrid driver really that complex? - it just creates a Linux bridge between the VIF and br-int. And I think the bulk of the complex code for the iptables rules is being reused by other plugins, like the Linux bridge plugin.