Combining two panes into a single one?

I am working on a UI design which has two left panes: the first shows the type of project that the user works on (i.e. custom web application, Shopify, Wordpress and so on)... the second needs to be a combination of the Feature Sets and Features view below... allowing the user to programmatically drill down from a "Feature Sets" level to a "Features" level. It is almost like a shopping basket for all the users need but encapsulated in a small frame:

Any ideas on how I can accomplish this in a top down manner? I am thinking of using an accordion design pattern but would be open to other approaches.

Inspiration:

My UI:

Full inspiration here.