I'd love to have the ability to pre-assign domains to specific containers. Containers is a promising privacy tool, but the current containers feature is very hard to use. I keep forgetting to open tabs in the right container or right-clicking certain links to open the right container. It requires too much explicit commitment from users to live to the promise.
I'd rather pre-assign domains to a container so that a page from an assigned domain always opens in the right container without additional effort from the user. Example, assign facebook.com to the facebook container. Everytime I open a facebook link, the link automatically opens the page in the facebook container. Awesome.