Adding an already opened page to open in a specific container in future should be an option. As of now it only shows option to add to default continer.

Absolutely great add-on!
Should be a built-in (or added by default) feature.
It's my No#1 tool for keeping services separated from each other, e.g. Facebook separately contained, News websites with their tracking cookies separately contained, other user accounts/profiles in various other websites and/or services w/o any association between them all.
It's like having multiple, persistent "Private Browsing / Anonymous" tabs with the ability to define a persistent default container for every website (e.g. clicking a Facebook link will always open up in a pre-defined Facebook container tab).
I love it and use it all the time!

Very good idea, needs more work, but nevertheless extremely useful in 2019s internet

really needs to be a built in feature, love it!