Many a time I’ve been involved in building a custom WordPress theme, which relied or benefited from some plugins being installed and activated. I’ve always had an ad hoc solution to the problem, with my own installation scripts, WP-CLI mockery, etc. “Packaging third-party plugins with your WordPress theme using TGM Plugin Activation library” covers a much more elegant solution. I haven’t tried it yet, but it does look very promising for my next WordPress project.