In the past month I've configured a multiWAN setup, and have a suggestion for the MultiWAN Manager rules:
Instead of just the 'Source Address' for applying a rule to a device, it would be nice to alternately set a MAC address.
This way devices don't need a static IP address and if they are using IPv6, then rules could be applied easier.
I had temporarily set up load balancing using FreshTomato and found that's an option they have. It seemed like a more elegant solution and would be great to see it in OpenWrt.